Preheat oven to 325°F (160°C). Butter an 8-inch (20cm) square glass baking pan and set aside.
Place the butter and chocolate in a microwave-safe bowl. Microwave at half power in 30 second intervals, stirring between each heating. Continue until both are completely melted.
Sift the sugar and cocoa powder. Add to the butter/chocolate mixture. Beat in (by hand) the mascarpone, eggs, and vanilla extract until smooth. Fold in the flour and salt.
Pour the batter into pan and spread evenly.
Bake for 45-50 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pan on cooling rack.
To make the ganache:
Place the chopped chocolate in a small bowl. Set aside.
In a small saucepan, heat the butter and cream over medium heat, stirring constantly. When mixture is almost boiling, pour over the chocolate. Let stand for 30 seconds. Then, stir until smooth.
Pour the ganache over cooled brownies and spread evenly.
Allow the ganache to firm before cutting brownies. Brownies may be placed in refrigerator to expedite this.
Notes
Store in an airtight container at room temperature up to 3 days.