Preheat oven to 350°F. Grease an 8 x 8-inch metal pan and line with parchment paper, leaving an overhang for easy removal. Avoid foil or wax paper.
Whisk melted butter, granulated sugar, and brown sugar in a large bowl until smooth. Add eggs and vanilla and whisk until creamy.
Add flour, cocoa powder, and salt. Switch to a wooden spoon and stir until just combined. Stir in 1/2 cup mini chocolate chips. Do not overmix.
Scrape batter into prepared pan and smooth the top. Sprinkle remaining 1/4 cup chocolate chips over the top.
Bake 25–30 minutes, until just set to the touch. Do not overbake. Centers should stay soft and fudgy. Let brownies cool completely before adding the topping.
Add chocolate chips to a heat-safe bowl. Microwave cream for 1 minute, then pour over chocolate. Cover and let sit 4 minutes, then stir until smooth. Microwave in 15-second bursts if needed.
Pour topping over cooled brownies and spread evenly. Immediately add rainbow chips or sprinkles on top.
Refrigerate 1–2 hours, until topping is set. Lift brownies out using parchment. Cut with a sharp knife, wiping clean between cuts. Serve chilled.
Notes
Note 1: Use Dutch-process for the best flavor and texture. Spoon and level to measure or brownies may end up dry.Note 2: Baking time may vary with dark-colored metal or glass pans. Light-colored, shiny pans produce the best results since they heat more evenly.Storage: Store in an airtight container up to 4 days at room temp or 1 week in the fridge. Freeze individually wrapped brownies up to 3 months.