Preheat the oven to 350°F. Grease a 13x9x2-inch baking pan or two 8- or 9-inch square pans with cooking spray.

In a large microwave-safe bowl, melt the butter. 2 to 2-1/2 minutes on medium (50%) or until melted Mix in the sugar and vanilla extract. Add the eggs one at a time, beating well after each addition with a spoon. Mix in the cocoa until well combined. Mix in the flour, baking powder, and salt. If desired, add nuts. Pour batter into prepared pan(s).

Bake for 30 to 35 minutes in a 13 x 9 x 2-inch pan (20 to 22 minutes in an 8- or 9-inch pan), or until brownies begin to pull away from pan sides. Allow to cool completely in the pan on a wire rack. If desired, prepare and frost the brownie with Creamy Brownie Frosting. If desired, top frosted brownies with milk chocolate chips. Make into bars.