Make cake: Microwave unsweetened chocolate in a small microwavable bowl for 2 1/2 minutes at medium (50%) power. Stir; cook in 30-second increments until melted. Reserve.
Preheat oven to 350°F. Grease a 13-by-9-inch metal cake pan; line it with greased parchment paper.
Cream butter and sugar with an electric mixer on high speed until fluffy, about 5 minutes. Beat in eggs one at a time, beating well after each addition. Add cocoa, baking powder, vanilla extract and salt; mix at low speed until blended. Mix flour in at low speed until just incorporated; mix in reserved chocolate until just incorporated.
Batter will be thick. Scrape it into prepared pan and smooth top. Press pitted cherries into surface of batter at 1-inch intervals; bake 25 to 30 minutes, until a toothpick inserted into center comes out clean. (Do not stick pick into a cherry.) Let cake cool for 10 minutes, invert pan onto a tray, then invert cake onto cooling rack; cool completely.
Make frosting: Melt semisweet chocolate in 1 cup of cream in a small saucepan over medium heat; stir until smooth. Scrape into a bowl, then stir in second cup cream and chill for at least 2 hours. Beat cold chocolate cream with an electric mixer until thick (do not overbeat or mixture may become grainy).
Assemble cakes: Beat 1/2 cup cream with confectioners' sugar until stiff but not dry. Using a 2 3/4-inch-diameter cookie cutter, cut out 8 rounds of cake; arrange on a platter. Frost cakes, dollop with whipped cream and top with a fresh cherry.