Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper for easy removal.

In a large mixing bowl, combine the melted butter and granulated sugar. Whisk until smooth and glossy, which should take about 1-2 minutes.

Stir in the vanilla extract, followed by the eggs, mixing well until fully incorporated. The mixture should be thick and slightly glossy.

In a separate bowl, sift together the all-purpose flour, cocoa powder, salt, and baking powder. Gradually add this dry mix to the wet ingredients, stirring just until combined. Be careful not to overmix; the batter should be thick and slightly fudgy.

Pour the brownie batter into the prepared baking dish, smoothing the surface evenly.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs—this ensures the brownies remain fudgy.

Allow the brownies to cool completely in the pan before icing.

For the icing, combine the cocoa powder, melted butter, powdered sugar, milk, and vanilla extract in a mixing bowl. Whisk until smooth and spreadable.

Once the brownies are cooled, spread the chocolate icing evenly over the top. Then, generously sprinkle your rainbow sprinkles on top while the icing is still soft.

After the icing has set for about 15-20 minutes, slice the brownies into squares and enjoy!