Step 1

Position a rack in the lower third of the oven and preheat the oven to 325°F. Line the bottom and sides of an 8×8-inch square baking pan with parchment paper or foil, leaving an overhang on two opposite sides.

Step 2

Combine the butter, sugar, cocoa, and salt in a medium heatproof bowl and microwave for 90 seconds. Remove and stir until the butter is melted and the mixture is smooth. Set aside briefly until the mixture is only warm, not hot. It looks fairly gritty at this point, but don’t fret — it smoothes out once the eggs and flour are added.

Step 3

Stir in the vanilla with a wooden spoon. Add the eggs one at a time, stirring vigorously after each one. When the batter looks thick, shiny, and well blended, add the flour and stir until you cannot see it any longer, then beat vigorously for 40 strokes with the wooden spoon or a rubber spatula.

Step 4

Stir in the butterscotch chips, if using. Spread evenly in the lined pan.

Step 5

Bake until a toothpick plunged into the center emerges slightly moist with batter, 25 to 30 minutes. Let cool completely on a rack before cutting and serving.