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, glossy, and slightly thickened.
04 - Sift the cocoa powder, flour, and salt directly into the wet mixture. Fold gently with a spatula until just combined — do not overmix to keep the texture tender.
05 - Pour the batter into the prepared baking pan and spread it into an even layer using a spatula.
06 - Bake for 22–25 minutes, or until the center is just set but still slightly moist. Avoid overbaking for a fudgy result.
07 - Let the brownies cool completely in the pan on a wire rack before adding the ganache.
08 - Place the finely chopped chocolate in a heatproof bowl. Heat the heavy cream in a small saucepan until just simmering, then pour it over the chocolate. Let it sit for 2 minutes, add the butter, then stir gently until the ganache is smooth and glossy.
09 - Pour the ganache over the cooled brownies and spread it into an even, smooth layer.
10 - Immediately scatter candy-coated chocolate chips or rainbow sprinkles over the ganache. Gently press them down so they adhere.
11 - Refrigerate the brownies for at least 20 minutes to allow the ganache to firm up.
12 - Lift the brownies out of the pan using the parchment paper overhang. Slice into 12 even bars and serve.