Soft butter, for greasing the pan
Flour, for dusting the buttered pan
4 large eggs
1 cup sugar, sifted
1 cup brown sugar, sifted
8 ounces melted butter
1 1/4 cups cocoa, sifted
2 teaspoons vanilla extract
1/2 cup flour, sifted
1/2 teaspoon kosher salt
Preheat the oven to 350 degrees F. Butter and flour an 8-inch square pan. In a mixer fitted with a whisk attachment, beat the eggs at medium speed until fluffy and light yellow. Add both sugars. Add remaining ingredients, and mix to combine.
Pour the batter into a greased and floured 8-inch square pan and bake for 45 minutes. Check for doneness with the tried-and-true toothpick method: a toothpick inserted into the center of the pan should come out clean. When it's done, remove to a rack to cool. Resist the temptation to cut into it until it's mostly cool.