12 tablespoons unsalted butter, melted, plus more for pans
2 cups all-purpose flour, plus more for pans
1 1/2 cups sugar
3/4 cup unsweetened cocoa powder
1/2 teaspoon salt
1 1/2 teaspoons baking powder
3/4 teaspoon baking soda
1 large whole egg, room temperature
1 large egg white, room temperature
1 teaspoon pure vanilla extract
1 1/3 cups strong, hot coffee
Heat oven to 325 degrees. Butter two 8-inch-square baking pans. Line with parchment, and butter again. Flour pans, tapping out excess; set aside.
In the bowl of an electric mixer fitted with the whisk attachment, combine all the ingredients except the hot coffee. Mix ingredients on low speed until combined, about 1 minute. Slowly add the coffee. Mix until smooth, about 1 minute. Pour batter into prepared pans; smooth tops with an offset spatula.
Bake until a cake tester inserted into the middle comes out clean, 25 to 30 minutes. Transfer the pans to wire rack to cool. Turn out the cakes, and wrap them in plastic wrap until ready to use.