01 - Preheat oven to 350°F. Line an 8x8 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 and glossy.
04 - Sift in the cocoa powder, flour, and salt. Fold gently with a spatula until just combined—do not overmix to keep the brownies tender.
05 - Pour the batter into the prepared pan and spread it evenly. Bake for 22–25 minutes, or until the center is just set but still slightly moist. Avoid overbaking.
06 - Remove from the oven and let the brownies cool completely in the pan on a wire rack.
07 - Place the finely chopped chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream until just simmering—do not boil. Pour the hot cream over the chocolate and let it sit undisturbed for 2 minutes. Add the butter, then stir gently from the center outward until the ganache is smooth and glossy.
08 - Spread the ganache evenly over the cooled brownies. Immediately scatter the candy-coated chocolate chips or rainbow sprinkles on top, pressing gently so they adhere to the surface.
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.