01 - Preheat oven to 350°F. Line an 8×8-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a large mixing bowl, whisk together the melted butter, granulated sugar, and packed brown sugar until smooth and glossy.
03 - Add the eggs and vanilla extract to the sugar mixture, whisking vigorously until well combined and the batter appears slightly lightened in color.
04 - Sift the cocoa powder, all-purpose flour, and fine sea salt directly into the wet mixture. Gently fold with a spatula until just combined, being careful not to overmix.
05 - Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 23–25 minutes, until the edges are set but the center still looks slightly underbaked. Cool completely in the pan on a wire rack.
06 - Place the chopped semi-sweet chocolate and butter in a heatproof bowl. Heat the heavy cream in a small saucepan over medium heat until just steaming, then pour it over the chocolate. Let sit for 2 minutes.
07 - Stir the chocolate-cream mixture gently until completely smooth and glossy. Spread the ganache evenly over the cooled brownie base.
08 - Sprinkle the rainbow candy-coated chocolate chips evenly over the ganache layer. Refrigerate for at least 30 minutes, or until the ganache is fully set.
09 - Lift the brownie slab from the pan using the parchment overhang. Slice into 12 even bars with a sharp knife and serve at your preferred temperature.