Melt ¾ cup butter and ½ cup chocolate chips in a medium saucepan over medium-low heat, stirring occasionally, until fully smooth, about 3-4 minutes. Remove from heat.

Add ½ cup granulated sugar, ½ cup brown sugar, and ⅓ cup cocoa powder directly to the pot and whisk vigorously until smooth and glossy.

Add 2 eggs and 1 egg yolk and whisk until you have a thick and glossy batter.

Add ¾ cup flour and ½ teaspoon salt, and whisk in gently, just until all the flour is incorporated. Transfer the batter to a parchment-lined 9x9-inch baking dish and smooth into an even layer. Bake at 350°F for 30-35 minutes, until the edges are set and a toothpick inserted in the center comes out with just a few crumbs. Let the brownie cool completely in the pan.

Heat ½ cup heavy cream in a small saucepan over medium heat until steaming. Remove from heat and add ¾ cup of chocolate chips. Let that sit for 5 minutes, then whisk the cream and chocolate chips until you have a smooth frosting.

Pour the frosting over the cooled brownies and spread into an even layer. Top with rainbow sprinkles and refrigerate for 30 minutes - 1 hour the frosting has hardened slightly. Slice into 16 brownies and enjoy.