Preheat oven to 325°F; have ready a buttered 8-inch square glass cake pan.
First, prepare the brownies.
In a small saucepan, melt the butter and bring it to just below a boil; have the chopped chocolate in a mixing bowl--pour the hot butter over the chocolate and let stand for 30 seconds.
Stir until chocolate is completely melted.
Sift in the sugar and cocoa powder (I do this through a sieve, not with a sifter).
With a wooden spoon, beat in the mascarpone, eggs (30 minutes on the counter brings them to room temperature) and vanilla, mixing until smooth.
Gently fold the flour and salt into the batter.
Pour batter into prepared pan and spread evenly--this is important since, if the batter isn't spread evenly, it won't bake evenly.
Place into preheated oven and bake for 45 to 50 minutes, or until a tester comes out clean (45 minutes did it for me).
Place pan on a cooling rack and let brownies cool for 10-15 minutes, while you now make the ganache.
Place chopped chocolate in a mixing bowl; in a small saucepan, bring the cream and the butter to just below the boiling point, over medium heat.
Pour this hot cream-butter mixture over the chocolate and let stand for 30 seconds, then stir until smooth; ganache is now ready to use and can be spread over the brownies.
Should you wish to wait a bit, make sure the ganache is warm when you spread over the brownies, as it does firm up which makes spreading hard to do.
Don't cut into the brownies until ganache has firmed up; I find it best to put the brownies into the fridge to speed this along; once the ganache is firm the brownies do not need to be kept in the fridge, though.