Preheat your oven to 325°F (160°C) and grease an 8x8-inch baking dish and line it with parchment paper.

In a large bowl, whisk together the melted butter, granulated sugar, and light brown sugar until smooth. Add the sifted cocoa powder, vanilla extract, eggs, yolk, and corn syrup. Mix until the batter is fully combined and no lumps remain.

Gradually fold in the flour, cornstarch, and salt using a spatula. Stir gently until the batter is smooth and no dry streaks remain, but be careful not to overmix.

Pour the batter into the prepared baking dish, spreading it evenly with a spatula to smooth the top.

Bake for 25–28 minutes, or until the edges are set and the center is slightly fudgy. A toothpick inserted 1 inch from the edge should come out with a few crumbs. Allow the brownies to cool completely in the pan before adding the frosting.

For the frosting, heat the heavy cream in a small saucepan over medium heat until it just starts to simmer, do not boil. Remove from heat and pour over the semi-sweet chocolate chips in a heatproof bowl.

Whisk the mixture until the chocolate is fully melted and the frosting is smooth and glossy. Immediately spread this over the cooled brownies in an even layer.

Top with candy-coated chocolate chips, pressing them lightly into the frosting to ensure they stick. Chill the brownies in the refrigerator for at least 30 minutes or until the frosting is firm. Slice into 12 squares and serve. Enjoy!