Pour melted butter into dry ingredients and stir together until really well-combined with a rubber spatula. Add eggs one at a time, combining well after each addition.
In a large mixing bowl, whisk together flour, baking powder, salt, and sugars until well-combined.
Preheat oven to 350º F. Spray a 9×13 baking dish with nonstick baking spray or spread with a thin coating of butter and then lightly flour the pan, removing any excess flour.