Preheat oven to 350 F. In microwave, melt 1 cup butter (2 sticks) and 1 cup semi-sweet chocolate pieces. Let cool a bit.
Drain a jar of sour cherries. I used frozen ones that were defrosted. Make sure they are well-drained.
Separate 6 eggs. Beat the egg whites until soft peaks form; set aside. In mixer bowl, combine the 6 egg yolks and 1 cup sugar. Beat until eggs are lemon-colored. Beat in 1–1/4 cups flour and 1/2 tsp salt until mixed through. Add 1 tsp vanilla extract, chocolate mixture and egg whites. (I made no effort to fold them in carefully.)
In a buttered 9–1/2 inch spring-form pan, pour in the batter. (I only had an 8–1/2 and so I put some of the batter in ramekins to make small cakes. I’m sure you could use a 10–1/2-inch pan.) Place the drained cherries on top of the cake. Bake until a toothpick inserted comes out clean, about 65 to 75 minutes, depending on the pan. (I suspect in a 10–1/2 inch pan this might be done after as little as 55 minutes.)
Remove from oven, let cool in pan. After 15 minutes run a knife around the outside of the pan but let it complete cooling in the pan.
Serve with whipped cream.