Preheat oven to 350°F.
Line an 8 inch x 8 inch baking pan with parchment paper. Set aside.
In a mixing bowl, whisk together cocoa powder, flour, and salt.
Then, add the melted butter, white sugar, brown sugar, eggs, and vanilla extract. Combine together with a rubber spatula but do not over mix.
Transfer into baking pan and pop in the oven to bake for 20 to 25 minutes. Do not overcook.
While it bakes, make your frosting. Add the melted unsalted butter, sugar, heavy cream, and chocolate chips/chunks to a microwave-safe bowl. Microwave in 15 second intervals, mixing in between until the chocolate is melted (only took 30 seconds for me).
Take the brownies out of the oven and allow to cool to room temperature.
Add the frosting on top and chill for a few minutes in the fridge until the frosting is firm and not runny.
Sprinkle on the rainbow chips and cut into rectangles!