Preheat oven to 350. Prepare an 8 in baking pan with butter and flour.
Melt the butter and chocolate together in a microwave. Cool slightly. Stir in the brown sugar, sugar and vanilla. Beat in the eggs until light and fluffy. Add the flour, cocoa powder, and salt and stir until just combined, being careful not to over-mix.
Pour the batter into the prepared pan and bake until the brownies are set in the middle, about 30 minutes.