½ cup (1½ oz.) cocoa powder
6 tablespoons boiling water
2 oz. (about ⅓ cup) semisweet chocolate, melted
1 large egg + 1 egg yolk (reserve the white for the filling, below)
½ cup sour cream
1 tablespoon vanilla extract
1 cup unbleached all-purpose flour
½ teaspoon baking soda
¼ teaspoon baking powder
¼ teaspoon kosher or fine-grained sea salt
1 cup granulated sugar
8 tablespoons chilled unsalted butter, cut into ½-inch cubes
Marshmallow Cream Filling:
⅓ cup granulated sugar
1½ teaspoons light corn syrup or Lyle’s golden syrup
2 tablespoons water
2 large egg whites
1 14.5-oz. can unsweetened full-fat coconut milk, refrigerated overnight
1 cup pitted sour cherries (fresh, jarred, or thawed from frozen)
Make the cake:
Preheat the oven to 325˚F and spritz a 9- by 13-inch metal baking pan with nonstick baking spray.
Whisk the cocoa powder and boiling water together in a medium mixing bowl to make a paste, then stir in the melted chocolate. Reserve.
Whisk the eggs, sour cream, and vanilla together in a small mixing bowl. Reserve.
Whisk the flour, baking soda, baking powder, and salt together in a small mixing bowl. Reserve.
Beat the butter and sugar together in the bowl of a stand mixer fitted with the paddle attachment on medium speed for about 3 minutes, until the butter is pale and fluffy.
Scrape the reserved cocoa paste into the butter and stir on low speed until fully incorporated.
Add a third of the flour and stir until just incorporated, then half the egg and sour cream mixture. Repeat with a third more of the flour and the remaining half of the egg mixture, then stir in the remaining flour.
Scrape the batter into the prepared baking pan, using a silicone spatula to distribute it in a thin, even layer.
Bake for 20-25 minutes, until a cake tester or toothpick inserted into the center of the pan comes out clean.
Cool the cake in the pan on a wire rack for 10 minutes, then invert the pan onto the rack to remove the cake. Cool completely.
To fill the cakes choose one of the following cream fillings (or both!):
Make the marshmallow cream filling:
Stir the sugar, corn syrup, and water together in a small, high-sided saucepan over medium low heat just until the sugar has fully dissolved and the liquid no longer feels granular.
Clip a candy thermometer to the side of the pan and bring the liquid to a boil without stirring. Continue to heat until the sugar syrup reaches 235˚-240˚F (soft-ball stage).
Whip the egg whites with an electric hand mixer or a stand mixer fitted with the whisk attachment on medium speed just until soft peaks form. Stop the mixer and, just before the sugar syrup reaches soft-ball stage, restart the mixer on low speed and carefully drizzle the syrup into the egg whites.
Increase the mixer speed to medium-high and whip for 4-5 minutes more until the filling is thick, shiny, and white, forming stiff peaks.
Make the cherry-coconut filling:
Open the can of coconut milk from the bottom of the can—the milk will have separated in the refrigerator into liquid and solid layers.
Pour the liquid into a small saucepan and add the cherries. Bring to a simmer and cook for about 15 minutes, until the cherries have absorbed a good deal of the coconut liquid. Remove from the heat and strain, then coarsely chop the coconut-infused cherries.
Whip the solid, cold coconut cream on medium-high speed with an electric hand mixer or stand mixer fitted with the whisk attachment for 2-3 minutes until fluffy and creamy.
Put it all together:
Slicing from the long side of the cake, divide the cake into 6 2- by 9-inch strips, then slice each strip into 5 rectangles each 1½ inches long. You’ll have 30 rectangles total once the entire cake is cut.
Spread your filling(s) of choice in thin layers on half the rectangles, then top with the remaining rectangles to make sandwich cakes.
Cakes will keep, refrigerated, in an airtight container for up to 3 days.