Preheat your oven to 350°F (175°C). This ensures that your brownies bake evenly and rise perfectly.
Line a 9x13 inch baking pan with parchment paper, leaving some overhang on the sides for easy removal later.
In a double boiler or microwave-safe bowl, combine 18 ounces of semi-sweet chocolate chips and 9 tablespoons of cubed unsalted butter. Stir occasionally until melted and smooth.
In a large mixing bowl, whisk together the melted chocolate mixture with 1 cup + 2 tablespoons granulated sugar and 1/2 tablespoon vanilla extract. Allow it to cool slightly before adding the eggs.
Beat in the 4 large eggs and 1 large egg yolk, one at a time, mixing well after each addition.
In a separate bowl, sift together 6 tablespoons Dutch process cocoa powder, 1/2 cup cornstarch, and 3/4 teaspoons salt. Gradually add this mixture to the wet ingredients, stirring until just combined.
Gently fold in 1-1/2 cups of chocolate chips.
Pour the brownie batter into the prepared baking pan, smoothing it out evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
While the brownies are baking, make the chocolate ganache by combining the remaining 1-1/2 cups of semi-sweet chocolate chips with 1/2 cup of heavy cream in a small saucepan over low heat. Stir until melted and smooth.
Once the brownies are done baking, allow them to cool in the pan for about 10 minutes. Pour the chocolate ganache over the top, spreading it evenly.
Before the ganache sets, sprinkle the Wilton rainbow chip crunch or mini M&Ms over the top. Allow the brownies to cool completely before lifting them out using the parchment paper.
Cut the brownies into squares and serve.