Preheat oven to 350 degrees and line a 9x13" baking dish with parchment paper. Mist with baking spray and set aside.

Beat melted butter, sugars, and vanilla in a large bowl using an electric mixer or stand mixer.

Add eggs, one at a time, continuing to mix until well incorporated.

In a separate mixing bowl, whisk together flour, cocoa powder, baking powder, cornstarch, and salt.

Gradually add the flour mixture to your batter, mixing on low speed. Alternate adding flour and milk, until everything is combined. Your batter should be fairly thick.

Scoop batter into your baking pan and spread with a spatula as evenly as possible.

Bake at 350°F for 18-25 minutes, taking care not to overcook.

Remove from oven and allow to cool in the pan.

Heat heavy whipping cream in a saucepan heating until simmering. Remove from heat and add chocolate chips. Allow them to sit in the hot cream for a couple minutes, then whisk until melted and smooth.

Pour ganache over the top of the brownies and smooth with a spatula.

Sprinkle rainbow chips over the top of the brownies, while frosting is still wet.

Allow to set, then slice and enjoy!