01 -
Position oven rack in middle and preheat to 325°F (162°C). Line 8-inch square pan with parchment paper, leaving overhang.
02 -
In medium saucepan, melt butter over medium-low heat until liquefied. Remove from heat before browning.
03 -
While butter is hot, stir in sugar, cocoa powder, vanilla, and salt until smooth. Mixture may be gritty.
04 -
Let mixture cool until warm to touch, about 5-10 minutes.
05 -
Add cold eggs one at a time, stirring vigorously after each until batter is thick and well blended.
06 -
Gradually add flour, beating with wooden spoon for 40-50 strokes until batter pulls away from sides.
07 -
Fold in nuts or chocolate chips if using.
08 -
Spread thick batter in prepared pan. Bake 20-30 minutes until edges are dry but center slightly underbaked.
09 -
Cool completely in pan. Chill 1-2 hours for clean slices. Cut into 16 squares.