Begin by preheating your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper.

Melt 1 cup of unsalted butter in a medium saucepan over low heat. Remove from heat and whisk in 1 cup granulated sugar and 1/2 cup brown sugar until smooth.

Allow the mixture to cool slightly, then incorporate 4 large eggs one at a time, whisking until fully integrated. Follow with 1 teaspoon of vanilla extract.

In a separate bowl, whisk together 1 cup cocoa powder, 1/2 cup flour, 1 teaspoon cornstarch, and 1/2 teaspoon salt. Gradually fold the dry ingredients into the wet mixture.

Gently fold in 1 cup of chopped semisweet chocolate. Pour the brownie batter into the prepared pan.

Place the pan in the preheated oven and bake for 25-30 minutes. Check for doneness with a toothpick.

Heat 1/2 cup heavy cream in a small saucepan and bring to a gentle simmer. Remove from heat and whisk in 8 ounces semisweet chocolate until smooth.

Once the brownies cool for 10 minutes, pour the ganache over the top and sprinkle with rainbow candy-coated chocolate chips.

Let the brownies cool completely in the pan for at least 1 hour. Lift them out using the parchment and cut into squares.