Chocolate Prune Sheet Cake

Sometimes, only a fudgy sheet cake with chocolate buttercream will do.

Hello friends! This week I’ve got a recipe for you - I REALLY love this one and I hope you do too.

But first to business! I’ll be slinging my final cakes the first two weekends of May. You can click here for early access to orders beginning at 10am on Wednesday, April 24th. Thank you so much for all your support! It’s wild being in the home stretch, and I hope to bake for as many of you as possible.

This is the first of two recipes I’m bringing you in partnership with my friends at California Prunes!

I’ve occasionally had folks apologize to me for serving store bought cakes at parties. I wish I could go back in time, gently hold them by the sides of their faces while maintaining eye contact, and truly, deeply convince them that: I do not care. Store bought cake is good! All cakes have their place! I grew up on the grocery store cakes my grandmother would bring home from her job in the checkout line at Graul’s Supermarket in Annapolis, sheet cakes piped with raucous neon roses. You know that buttercream - with the crusty exterior and velvety interior, scented with artificial vanilla extract. I still love that stuff.

When my pals at California Prunes came to me to develop a new recipe, I knew immediately that I wanted it to be simple to put together, and settled on a classic chocolate sheet cake, reminiscent of the ones I had as a kid. This one ups the ante with the inclusion of velvety prune puree and a hybrid Russian / American style cocoa buttercream. Together, they marry the nostalgia of that grocery-store sheet cake with the elegant flavor of prunes. This is the closest I may ever come to pairing fruit with chocolate (it’s just not my favorite combo!) but the winey, earthy prune puree adds just the right amount of acidity to the chocolate without veering into raspberry-coulis-with-molten-lava-cake territory. 

I don’t always work with American style buttercreams, but here the bracing sweetness is moderated by adding bittersweet cocoa and condensed milk, which adds a mild milkiness reminiscent of a chocolate milkshake. The cake itself just skirts the border between a devil’s food and a fudgy brownie. The inclusion of the prune puree will keep this cake moist for ages. And while I mainly suggest that folks temper their cakes before serving, this one is just begging to be served cold with an icy glass of milk. 

You can serve this in a simple single layer, swooped with the buttercream, or bake, slice into two pieces, and stack it for a fancier presentation (see the pics below for an example). If you’re doing the former, you can halve the buttercream recipe if you’re fond of just a thin layer of frosting - otherwise go ham and make the full amount for a thicker layer of buttercream. 

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Chocolate Prune Sheet Cake

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For the cake: 

1 ½ cups / 340 g water

1 cup, packed / 186 g california prunes 

16 tablespoons / 227 g unsalted butter

2 teaspoons vanilla

1/2 cup / 60 g bittersweet cocoa

4 oz / 113 g bittersweet chocolate, chopped 

¾ cup / 154 g brown sugar

1 ¼ cup / 250 g white sugar

2 whole eggs

1 teaspoon baking powder

½ teaspoon baking soda

3/4 teaspoon salt

2 cups + 1 tablespoon / 248 g all purpose flour

1/2 cup / 113 g buttermilk

Note: baking this cake at a slightly-lower-than usual temperature ensures that it bakes in a nice, flat layer, perfect for frosting.

Preheat the oven to 325. Grease a 9 x 13 pan (often called a brownie pan) with pan spray, line the bottom with parchment paper, and grease again. 

Combine the flour, baking powder, baking soda, and salt in a medium bowl and set aside. 

Simmer the water and prunes in a medium non-reactive pot for about five minutes at medium-high heat, or until the prunes are so soft you can easily squash them in half with a wooden spoon. Remove from the heat and let cool slightly before pureeing until very smooth (I use a magic bullet for smallish jobs like this as I abhor scraping small amounts from the bottom of a large blender). Return the prune puree to the pot and add the butter and vanilla extract. Warm over low heat until the butter is totally melted, whisking the mixture until it is totally combined. Add the chocolate and cocoa and continue to whisk over low heat until the chocolate has melted and the cocoa has bloomed and suffused the mixture, and there’s no trace of dry cocoa. Remove from the heat and add both sugars, whisking until smooth. Add the eggs and whisk until combined. Add the flour mixture and combine until no patches of dry flour remain. Finally, add the buttermilk and stir vigorously until it is fully incorporated.

Scrape the batter into the prepared pan and spread into an even layer, pushing the batter into the corner of the pans. Bake for 36-38 minutes, or until the edges of the cake have begun to shrink away from the sides of the pan, and a sharp knife inserted into the center comes out nearly clean, with perhaps a few stray crumbs. Allow to cool completely before frosting the cake. 

For a simple presentation, you can halve the following buttercream recipe and serve the cake in the pan, with the buttercream swooshed attractively on top (this will give you just enough to ice the surface of the cake in a thin layer). Alternatively, you can read on and prepare a more highly decorated cake.

To make a two-layer sheet cake, allow your cake to cool completely and unmold it from the pan. Slice it neatly in half so that you have two 4 ½ by 6 ½ inch layers. Place the bottom layer on a cake stand or serving platter and dollop about a cup of buttercream on top, smoothing it into an even layer with an offset spatula. Chill briefly. Place the second half on top and give your entire cake a quick crumb coat. Chill again for at least half an hour. When your crumb coat has chilled completely, frost the surfaces of your cake with an even, thin layer of buttercream. If you have fun piping tips to play around with, this is a great opportunity to go wild with them - if you don't like how the cake is going, you can always scrape it off and start over! 

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For the buttercream:

3 sticks / 336 grams butter, room temperature

1 can sweetened condensed milk

¼ cup - ½ cup / 30-60 grams bittersweet cocoa

½ - 1 cup / 62 - 125 grams confectioner’s sugar

Heavy pinch salt 

1 tablespoon vanilla extract

Note: You’ll see that both the suggested amounts of powdered sugar and cocoa in this recipe are listed as a range. Begin with the smaller amount, and if you prefer a sweeter buttercream or a deeper chocolate flavor, add the additional amount. For instance, when I used either Ghiradelli or Trader Joe’s cocoa, I found I preferred the lower amount of powdered sugar and the full amount of cocoa. However, when I tested the recipe with the bracingly dark Valrhona cocoa I use at work, I found I preferred the smaller amount of cocoa and the full amount of powdered sugar. This is a nice opportunity to really think about your preferred level of sweetness in a buttercream, and a fun way to experiment with the flavor of your preferred cocoa powder. That being said, this recipe isn’t meant to be the most chocolatey buttercream you’ve ever made - it’s designed to be reminiscent of the chocolate frosting on a child’s sheet cake.

Place your tempered butter in the bowl of a stand mixer fitted with a whisk attachment and beat on high for about five minutes or until it is very much lightened in color and very fluffy. Add the sweetened condensed milk and beat to combine. Sift the confectioner’s sugar, cocoa, and salt into the buttercream (taking into account the recipe note above). Beat once again until all the ingredients are fully incorporated, add the vanilla, and then beat for about five minutes more so that all of the confectioner's sugar is fully incorporated and there’s no trace of a powdery mouthfeel. Use immediately or store, refrigerated, for up to a week - you’ll need to let it come to room temperature and beat it once more before using. 

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