Preheat your oven to 350 degrees F. Coat the inside of an 8-inch x 8-inch baking pan with non-stick spray and then line it with parchment paper. Set aside.
In a large bowl, beat together the sugar, cocoa powder, butter, eggs, milk, corn syrup, and vanilla until smooth.
Gently fold in the flour and salt until the brownie batter is fully combined.
Pour the batter into the prepared baking dish. Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Meanwhile, prepare the frosting by warming the heavy cream in a small saucepan over medium heat. Bring almost to a simmer so that the cream is VERY warm. (DO NOT bring to a boil!) Remove the pan from the heat. Stir in chocolates until melted and smooth.
Pour the frosting over the cooked brownies. Then top with rainbow chip sprinkles and cool for 3 hours on a wire rack before slicing into 12 brownies.