Preparation

Make the cake:

• Position a rack in the middle of the oven and heat the oven to 350°F. Butter the bottom and sides of a 9×2-inch round cake pan. Line the bottom of the pan with parchment and butter the parchment.
• Fill a medium skillet with about 1/2 inch water and heat until just below a simmer. Put both the semisweet and unsweetened chocolate in a medium heatproof bowl and put the bowl in the barely simmering water. Stir until the chocolate is melted and smooth. Remove from the water bath and let cool slightly.
• Sift together the flour, cocoa powder, baking powder, baking soda, and salt. In an electric mixer (use the paddle attachment), beat the butter and sugar on medium speed until light and fluffy, 2  to 4 minutes. Mix in the slightly cooled melted chocolate on low speed just until incorporated. Increase the speed to medium and add the eggs one at a time, beating well after each addition. Scrape the bowl, add the vanilla, and beat on medium speed for another 1 minute. On low speed, mix in the sour cream just until it’s incorporated. Add the flour mixture (in three additions), alternating with the coffee (in two additions); scrape the bowl as needed. The batter will be very thick, like chocolate mousse or frosting.
• Scrape the cake batter into the prepared pan and smooth the top. Bake until the top feels firm and a toothpick inserted in the center comes out clean, about 35 minutes. The cake may sink a bit in the center, but that’s fine. Let the cake cool for 20 minutes in the baking pan on a wire rack. Using a small, sharp knife, loosen the sides of the cake from the pan, invert the cake onto the rack, and discard the paper liner. Let cool completely.

Prepare the cherries and kirsch syrup:

• Drain the cherries in a colander set over a large bowl (to catch the syrup) for 30 minutes. Reserve 1/2 cup of the syrup. Transfer the cherries to a small bowl, drizzle with 1 Tbs. of the kirsch, and set aside. Taste the syrup; it should be slightly tart and not too sweet. If necessary, stir in 1 to 2 tsp. sugar. Put the syrup in a small saucepan and simmer over medium heat until reduced by about half, about 3 minutes. Remove the pan from the heat and stir in the remaining 3 Tbs. kirsch. Set aside to cool.

Assemble the trifle:

• Put the cream, sugar, and kirsch in the large bowl of an electric mixer and whip on high speed until it holds firm peaks.
• Pick out the ten best-looking cherries and blot them dry with paper towels. With a long, serrated knife, cut the cooled cake vertically (all the way across the cake) into 1/2-inch slices.
  

• Line the bottom of a 2-1/2- to 3-quart glass bowl or trifle bowl with about a third of the cake slices to create an even layer. Don’t worry if the pieces break, as long as they fill in the spaces. Brush this layer of cake lightly with some of the kirsch syrup, top with a third of the whipped cream, and randomly nestle half of the remaining cherries into the cream. Sprinkle with a third of the chocolate shavings.

  

• Repeat with two more layers. On the top layer of cream, arrange the best-looking cherries in a ring near the rim of the bowl and scatter the chocolate shavings inside the cherry ring. Rrefrigerate for at least 30 minutes. Serve chilled.

Make Ahead Tips

The cake can be baked and the kirsch syrup prepared up to one day ahead. Wrap the cake in plastic and keep it at room temperature. Cover the syrup and keep refrigerated. Once assembled, the trifle can be refrigerated for up to 6 hours.