Preheat the oven to 350 degrees F.
Line a 9" x 9" square pan with a piece of parchment paper, leaving an overhang on both sides.
Coat the pan with cooking spray.
Add the granulated sugar and unsalted butter to a large mixing bowl and a hand mixer to beat together until light and fluffy, about 1 minute.
Add the eggs and vanilla extract to the bowl. Then beat for another 2 to 3 minutes until the mixture is pale yellow and thick.
Add the all-purpose flour, cocoa powder, and salt. Mix on low speed until ingredients are just incorporated.
Pour in the milk and beat until smooth.
Pour the batter into the prepared pan, and spread it into an even layer.
Bake for 20 to 25 minutes until a toothpick inserted into the center of the pan comes out clean or with just a few crumbs attached to it.
Let the brownies cool.
Place the heavy cream in a medium bowl. Microwave in 20 second increments until cream is very hot but not boiling.
Add the chopped semi-sweet chocolate and milk chocolate to the cream. Let sit for a minute, then stir until a smooth chocolate mixture forms.
Stir in the butter and vanilla extract until melted.
Pour the chocolate over the top of the brownies. Use a spatula to spread the chocolate into an even layer.
Add the rainbow chip sprinkles on top of the chocolate.
Chill the brownies until they set, about 2 hours to overnight.
Use the parchment to lift the brownies out of the pan.
Cut into squares and serve.
