Preheat your oven to 350°F (175°C). Grease an 8x8 inch baking pan or line it with parchment paper for easy removal.

In a medium bowl, melt the unsalted butter in the microwave or over low heat. Allow it to cool slightly.

Combine the granulated sugar and brown sugar with the melted butter, whisking until well blended.

Stir in the Dutch-process cocoa, salt, and vanilla extract until smoothly integrated.

Add the eggs one at a time, whisking thoroughly after each addition.

Gently fold in the all-purpose flour and cornstarch until just combined; it's okay if there are a few lumps.

Pour the batter into the prepared pan and spread it evenly to the corners.

Bake in the preheated oven for 20-25 minutes. Check for doneness by inserting a toothpick; it should come out with a few moist crumbs, indicating a fudgy interior.

While the brownies cool, heat heavy cream in a saucepan until it simmers. Remove from heat and stir in the chopped semisweet chocolate until the mixture is glossy and smooth, forming a ganache.

Pour the ganache over the cooled brownies, using a spatula to spread it evenly. Sprinkle rainbow candy-coated chocolate chips on top.

Let the brownies cool to room temperature before refrigerating for about 1 hour to allow the ganache to set.

Serve warm or with a scoop of ice cream for an extra treat.