Preheat oven to 325°F. Butter an 8-inch square pyrex pan.
Place the chopped chocolate in a mixing bowl; set aside. In a small glass bowl, melt the butter in a microwave, just until melted- don’t let it cook and bubble (if no microwave, melt quickly on the stove instead). Pour the butter over the chocolate and let stand for 30 seconds. Stir until the chocolate is completely melted and the butter is well incorporated. Sift in the sugar and cocoa powder.
With a wooden spoon, beat in the mascarpone, eggs and vanilla, mixing until smooth. Gently fold in the flour and salt.
Pour the batter into the prepared pan and spread evenly. Bake 45 to 50 minutes, or until a toothpick inserted into the center comes out clean. Place the pan on a cooling rack and let the brownies cool 10 to 15 minutes while you make the ganache.
PREPARE THE GANACHE FROSTING:
Place the chopped chocolate in a mixing bowl. In a small saucepan, bring the cream and butter to just below boiling point, over medium heat. Pour this hot mixture over the chocolate and let stand for 30 seconds, then stir until smooth. Pour the ganache over the brownies while still warm, and spread to cover evenly.
Let the ganache firm-up before cutting. It’s best to refrigerate them until quite firm. Once the ganache is firm and the brownies have been cut, they do not need to be kept in the refrigerator.