01 - Preheat 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, glossy, and slightly thickened.
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 texture 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 - Transfer the pan to a wire rack and let the brownies cool completely in the pan before adding the ganache.
07 - Place the chopped chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream until it just begins to simmer — do not boil. Pour the hot cream over the chocolate and let it sit for 2 minutes. Add the butter, then stir gently from the center outward until the ganache is smooth, glossy, and free of lumps.
08 - Pour the ganache over the cooled brownies and spread it into an even layer. Immediately scatter the candy-coated chocolate chips or rainbow sprinkles on top, pressing gently so they adhere to the ganache.
09 - Refrigerate the brownies for at least 20 minutes to allow the ganache to set firmly. Use the parchment overhang to lift the slab out of the pan, then slice into 12 bars and serve.