Preheat oven to 175°C (350°F) and line an 8x8 inch pan with parchment paper.
Melt butter and first batch of chocolate chips together over low heat, then whisk in sugar, eggs one at a time, and vanilla.
Stir in cocoa powder, flour, salt, and baking powder until just combined, then pour into pan and bake 20 to 22 minutes.
Cool brownies completely before topping.
Make ganache by pouring hot cream over chocolate chips, resting 2 minutes, then stirring until smooth.
Spread ganache over cooled brownies, scatter rainbow chips on top, and refrigerate 30 minutes before slicing.