Preheat oven to 350° F. Grease and flour a 9-inch springform pan. Set aside.

To prepare the cake, place unsweetened chocolate and cocoa powder in a small bowl. Carefully pour in boiling water and stir until melted and smooth. Set aside.

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

In a small bowl, combine sour cream, kirsch, and vanilla extract. Mix well and set aside.

In the bowl of a standing electric mixer, cream together butter and sugar until light and fluffy.

Add the egg to the butter mixture, beat well, and then add the egg yolk. Beat until blended. Add the chocolate mixture. Stir to mix.

Alternate adding the flour mixture and sour cream mixture to the batter in several additions. Stir until well combined.

Scrape the batter into prepared pan, smoothing the top to even out the batter. Bake for 30 to 35 minutes, until the cake is firm to the touch and a skewer inserted into the center comes out with just a few crumbs clinging to it.

Remove from the oven and cool the cake in the pan.

Toss pitted, sliced cherries with sugar and kirsch in a medium bowl. Set aside.

Whisk cream, powdered sugar, and vanilla in a large bowl until stiff peaks form.

Cut cake in half horizontally. Set half the cake, cut side up, on a cake stand and top with cherry mixture. Spread three-quarters of cream over cake and cherries, then top with remaining cake layer, cut side down. Swirl remaining cream on top. Garnish with whole cherries.

For easier slicing, refrigerate the prepared cake for about an hour before serving.