Instructions

1. 1
   Preheat oven to 325°F (160°C).
2. 2
   Line an 8x8-inch baking pan with parchment paper, leaving an overhang on the sides.
3. 3
   In a heatproof bowl set over simmering water (or in the microwave), melt the butter with sugar, cocoa powder, and salt.
4. 4
   Stir until warm and smooth.
5. 5
   Remove from heat and let cool slightly until warm (not hot).
6. 6
   Stir in the vanilla extract.
7. 7
   Add the eggs one at a time, stirring vigorously after each until the batter looks shiny and well blended.
8. 8
   Add the flour and stir until just combined, then beat strongly with a spatula for 40 strokes to develop the structure.
9. 9
   Fold in mix-ins if using.
10. 10
    Spread the batter evenly into the prepared pan.
11. 11
    Bake for 20 to 22 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
12. 12
    Do not overbake.
13. 13
    Let cool completely before slicing.