Preheat the oven and melt the butter in a in a small pan on the stove, but not completely.
In a bowl, crush the chocolate and pour the hot butter over it. Leave this to stand for 30 seconds and stir until the chocolate melts.
Sift the cocoa into the sugar. With a wooden spoon, mix the eggs (which have sat for at least 30 minutes at room temperature) with the mascarpone and vanilla. Stir until you get a smooth mixture. Add the flour and salt carefully.
Add the cocoa and sugar while stirring and gently begin pouring the entire mixture into the melted chocolate with the butter. Stir until the mixture becomes homogenized and pour it into a buttered, warmed baking dish, even it out thoroughly.
Bake the brownies with mascarpone for 45-50 minutes or until there's no crumbs stuck on your toothpick when you poke them with it.
Take out the finished cake and cool it in an oven dish with ice water while you prepare the glaze.
Crumble the chocolate for the glaze into a bowl, melt the butter for it in a pan and stir with the cream.
Pour the hot mixture over the chocolate and wait 30 seconds. Stir until homogeneous. Pour the warm glaze evenly over the cooled brownies. Leave it in the refrigerator, to cool completely. Cut into square pieces and serve.