01 - Preheat the oven to 350°F. Line an 8×8 inch 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 brown sugar until smooth and well blended.
03 - Add the eggs, egg yolk, and vanilla extract to the sugar mixture. Whisk vigorously until the batter becomes light in color and glossy in texture.
04 - Sift the cocoa powder, flour, and salt directly into the bowl. Fold gently with a spatula until just combined—avoid overmixing to keep the brownies tender.
05 - Pour the batter into the prepared pan and spread it into an even layer. Bake for 22–25 minutes, or until the center is just set but still slightly moist. Do not overbake.
06 - Remove from the oven and let the brownies cool completely in the pan on a wire rack.
07 - Place the finely chopped semi-sweet chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream until it just begins to simmer, then pour it over the chocolate. Let it sit for 2 minutes, then add the butter and stir gently until the ganache is smooth, glossy, and lump-free.
08 - Spread the ganache evenly over the cooled brownie base. Immediately scatter the candy-coated chocolate chips or rainbow sprinkles on top and gently press them into the ganache so they adhere.
09 - Refrigerate for at least 20 minutes to allow the ganache to set fully. Use the parchment overhang to lift the brownies from the pan, then slice into 12 bars and serve.