Line a 13x18-inch baking sheet with parchment and spray it with nonstick cooking spray. Preheat the oven to 350 F.
Add the butter and both sugars to the bowl of a large stand mixer fitted with a paddle attachment. Beat them together on medium-high speed, until light and fluffy, about 5 minutes. Add the eggs one at a time, beating well after each addition. Add the vanilla extract and mix it in.
In a separate bowl, whisk together the buttermilk, sour cream, and water (or coffee, if using). In a different bowl, sift together the flour, cocoa powder, baking soda, and salt. With the mixer running on low, add a quarter of the sifted flour mixture. When the flour streaks have almost all disappeared, add a third of the liquid to the mixing bowl. When that’s incorporated, continue to add the drys and wets in an alternating pattern, ending with the dry ingredients.
When the dry ingredients are nearly incorporated, stop the mixer. Scrape down the bottom and sides of the bowl with a rubber spatula, and finish mixing the cake by hand. Pour the batter into the prepared baking sheet, and spread it into a thin layer. Bake the cake for 15-18 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs attached. Cool the cake completely before using.
The cake can be made several days in advance and kept, well-wrapped and refrigerated or frozen, until ready to use.
To Make the Kirsch Syrup:
Combine the sugar and water in a small saucepan over medium heat. Stir while the sugar dissolves. Bring the liquid to a boil, then remove it from the heat. Let it cool to room temperature, then stir in the kirsch or cherry extract. Syrup can be made up to a week in advance and stored in the refrigerator until ready to use.
To Make the Chocolate Pastry Cream:
In a medium bowl, whisk together the yolks, cornstarch, and 1/4 cup of sugar. In a medium saucepan, combine the milk, remaining 1/4 cup sugar, and salt. Heat the milk over a medium burner until it just starts to boil. Start whisking the egg mixture, and while you're whisking, drizzle a little hot milk into the eggs. Continue to whisk and drizzle until you've added about half of the milk. Switch to whisking them milk, then pour the eggs into the milk mixture while whisking.
Return the pan to the burner and heat the cream, whisking constantly. Use a rubber spatula to periodically scrape the bottom of the pan so it doesn't scorch. Cook until the pastry cream thickens and starts a very gentle bubbling, then cook for about 2 minutes longer. Remove the pan from the heat and add the melted chocolate, whisking it until it's incorporated and the mixture is smooth. Finally, add the vanilla extract and butter and whisk them in.
Pour the cream through a wire mesh strainer into a bowl. It will be somewhat thick, so use a spatula to help work it through, straining out any clumps of egg that have developed. Press a layer of cling wrap directly on top of the pastry cream, and refrigerate until cold and firm, at least 2 hours.
First, make the chocolate shavings for the decorations. Hold the chocolate block in one hand, and scrape a vegetable peeler down the side with the other hand to make thin chocolate curls. Repeat until you've turned the whole chocolate block into shavings.
Combine the cream with the powdered sugar and vanilla, and whip on medium-high speed until it holds firm peaks. Set aside 1/2 cup of the cream in the refrigerator, and gently fold the rest of the whipped cream into the chocolate pastry cream.
Use a 3-inch cutter to cut 21 rounds out of the sheet cake. In order to get them all to fit, some of them may not be perfectly round—that's okay, it will all be covered up eventually! Cut 7 small circles of cardboard to the same size as the 3-inch cutter.
Place a cake round on a cardboard circle. Brush the top with some kirsch syrup. Spread a thin layer of pastry cream on top of the cake, and sprinkle the top with a layer of chopped cherries. Add a second cake layer, and repeat the process of Kirsch syrup, pastry cream, and cherries. Add a third cake layer on top, and spread a very thin layer of pastry cream on the sides and top of the cake, to lock the crumbs in. Refrigerate the cake while you assemble the rest of them in the same way. Let the assembled cakes chill for 20-30 minutes, to get them a little firmer before finishing them.
Once the mini cakes have firmed up a bit, add a second, thicker layer of chocolate pastry cream on the sides and top of the cake. Press chocolate shavings all over the top and sides. One all of the cakes are assembled, pipe a swirl of whipped cream on top of each, and finish with a fresh cherry on top. If not serving immediately, keep the cakes in the refrigerator until shortly before serving. They do keep well, so they can be made a day or two in advance.