Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.

In a large mixing bowl, melt 1 cup of unsalted butter and stir in 1 cup granulated sugar and 1 cup brown sugar until fully combined.

Add 4 eggs one at a time and mix well, followed by 1 teaspoon of vanilla extract.

Gently fold in 1 cup all-purpose flour, 1 cup cocoa powder, and a pinch of salt until the mixture is smooth but not overmixed.

Pour the brownie batter into the prepared pan and bake for 25-30 minutes.

Let the brownies cool in the pan for about 15 minutes before transferring to a wire rack to cool completely.

In a large bowl, beat 1 cup of unsalted butter until creamy. Gradually add 4 cups of powdered sugar, mixing on low until combined.

Pour in 2 tablespoons of heavy cream and continue to beat until fluffy—about 3-5 minutes.

Divide the frosting into bowls and use gel food coloring to create vibrant colors.

Spread a generous layer of frosting over the cooled brownies, dollop colored frosting, and swirl together.

Sprinkle rainbow sprinkles on top, allow to set for about 10 minutes, then slice and serve.