Place butter and chocolate in a double boiler and melt together over low heat. Remove the pan from the heat and stir until smooth and thoroughly blended. Let it cool to room temperature.
Preheat the oven to 375℉ when using a metal pan or 350℉ if using a glass pan
Add the eggs, vanilla and salt to the bowl of a standing mixer. Beat together until it is very thick and light in color. Gradually add the sugar and continue beating at medium-high speed until the eggs reach the consistency of soft peak whip cream- about 8-10 minutes in total. The trail of the eggs will remain on the surface for 15 seconds when you lift the beater.
Pour the melted chocolate mixture around the edge of the eggs mixture and gently fold just to combine. Sift the flour and fold in gently.
Pour the batter into the prepared pan, and carefully smooth the top. Bake at the center of the oven for 35 minutes or until the top is crisp and dry but wet inside.
Let the brownie cool completely in the pan before cutting.
