Butter a 8×8-inch baking pan and line with buttered parchment paper. Preheat oven to 350 degrees. In top of a double boiler set over barely simmering water, or on low power in a microwave, melt 1 stick butter and chocolate together. Cool slightly. In a large bowl, whisk eggs until frothy.
Whisk in salt, both sugars and vanilla.
Add the chocolate mixture to the bowl, stir to combine. Sift in the flour and cocoa, fold together until barely combined. Fold in the cherries.
Spread batter into prepared pan.
Bake for 30 minutes or until shiny and beginning to crack on top. Cool in pan on rack.
