Preheat your oven to 350°F. Line a 9x13-inch baking dish with parchment paper or foil and lightly grease.
In a large bowl, whisk together melted and cooled butter and sugar until well-mixed. Add eggs and vanilla extract and beat until well-combined.
Add flour, sifted cocoa powder, baking powder and salt. Use a wooden spoon or spatula to stir until combined.
Transfer batter to prepared baking dish, smoothing out the top slightly.
Bake for 27-35 minutes, until a toothpick or cake tester inserted into the center of the brownies comes out with just a few moist crumbs.
Cool completely before frosting.
Chocolate Frosting
In a large bowl, beat butter until light and fluffy. Add powdered sugar, cocoa powder, vanilla, optional espresso powder, and salt. Pour in milk as you beat the frosting until it reaches your desired consistency, adding more milk by the 1/2 teaspoon if necessary.
Spread frosting over completely cooled brownies and add sprinkles if desired. Slice and enjoy!