Preheat oven to 350°F (175°C) and lightly grease or line a 9x13-inch baking pan with parchment paper.
In a large bowl, whisk melted butter with granulated and brown sugars until smooth.
Add eggs and vanilla extract, mixing until the batter is glossy and slightly thickened.
Sift in cocoa powder, flour, salt, and baking powder, then gently stir until just combined.
Fold in chocolate chips if using.
Pour the batter evenly into the prepared pan and smooth the surface with a spatula.
Bake for 25–30 minutes until a toothpick inserted near the center comes out with a few moist crumbs.
Allow brownies to cool completely in the pan before frosting.
Beat softened butter until creamy, then add cocoa powder and mix until smooth.
Gradually add powdered sugar alternating with milk, then mix in vanilla extract.
Beat frosting until fluffy and spreadable.
Spread frosting evenly over cooled brownies with an offset spatula or knife.
Immediately sprinkle pastel sprinkles evenly over the frosting before it sets.
Slice brownies into neat squares using a sharp knife, wiping clean between cuts.
Serve and enjoy your frosted sprinkle brownies!