Preheat oven to 350 degrees F. Line a 13×9 inch metal pan with foil and spray foil with cooking spray. Alternatively, you may use parchment. These are pretty sticky at first!

Melt butter and chocolate in a 3-qt heavy saucepan over low heat, stirring, until smooth. Add 1 cup of the sugar to the hot chocolate mixture. Remove from heat and let cool to lukewarm.

Whisk in remaining 1 cup sugar and vanilla. Whisk in eggs 1 at a time until mixture is glossy and smooth. Whisk in salt and cocoa powder, then add flour and stir until it disappears (do not overbeat).

Spread in pan and bake until a pick inserted in center comes out with crumbs, 25 to 35 minutes. Cool to room temperature then chill for an hour or more. Lift from pan, place on a cutting board, cut into squares.

Makes 1 13×9 inch pan (yield depends on how big you cut your brownies, but they are rich, so small is fine.