For the brownies: – 1 1/2 sticks (12 Tablespoons, 3/4 cup) unsalted butter – 1 cup brown sugar – 3/4 cup white sugar – 3 egg – 1 egg yolk – 2 teaspoons vanilla – 1 cup sifted cocoa powder – 1/3 cup all purpose flour – 1/2 teaspoon salt – 1/2 teaspoon baking powder – ½ cup espresso or strong brewed coffee – 1 Tablespoon dark rum For the filling: – 3 egg yolk – 2 Tablespoons dark rum – 3 Tablespoons sugar – 8 ounces mascarpone cheese, at room temperature – 1 cup cold heavy whipping cream For the topping: – Cocoa powder – Dark chocolate bar