Preheat oven to 350°F and line an 8×8-inch metal pan with parchment. Whisk oil and both sugars, then add eggs and vanilla and whisk vigorously for 1 minute.

Sift in cocoa, flour, and salt and fold with a spatula just until no dry streaks remain, about 12 to 15 strokes.

Spread the thick batter into the pan and smooth the top. Press any mix-ins into the surface if using.

Bake 22 to 25 minutes until the top is glossy and crackled and a toothpick 2 inches from the edge comes out with moist crumbs.

Cool in the pan 10 minutes, lift out with parchment, and cool completely on a rack. Slice into 16 squares with a sharp knife.