Whisk in the chocolate-butter mixture until glossy and thick. Sprinkle the flour and salt into the bowl and stir until just incorporated. Spread the brownie batter in the prepared baking pan.
Preheat oven to 180°C and line a 9x10 inch pan with parchment paper. Melt butter, then mix with brown sugar, regular sugar, salt, and vanilla using a whisk or electric mixer. Add eggs and beat until ...