In a medium mixing bowl, whisk together the flour, ⅔ cup sugar, ¼ cup cocoa powder, baking powder, and salt until combined. In a separate bowl, whisk together milk, butter, and vanilla until smooth. Add the wet mixture to the dry mixture and stir to combine into a thick batter. Pour into the prepared baking dish.
1 cup all-purpose flour, 1⅔ cups granulated sugar, 6 tablespoons unsweetened cocoa powder, 2 teaspoons baking powder, ½ teaspoon kosher salt, ½ cup milk, 6 tablespoons unsalted butter, 1 teaspoon pure vanilla extract