Instructions

• Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper and spray the parchment with nonstick cooking spray; set the pan aside.
• In a large mixing bowl, add the melted butter, granulated white sugar, brown sugar, the 3 eggs and the egg yolk, vanilla extract, and vegetable oil. Whisk or beat until the mixture is smooth and evenly combined; set this wet mixture aside.
• In a separate bowl, whisk together the all-purpose flour, cornstarch, unsweetened cocoa powder, salt, and baking soda until evenly distributed.
• Pour the dry ingredients into the wet ingredients and stir until just combined and no large streaks of flour remain. Do not overmix.
• Transfer the batter to the prepared 9×13 pan and spread it into an even layer.
• Bake in the preheated oven for about 25 minutes, or until the edges begin to brown and a toothpick inserted near the center comes out with a few moist crumbs (not wet batter).
• Remove the pan from the oven and allow the brownies to cool completely in the pan on a wire rack.
• While the brownies cool, prepare the topping: place the semi-sweet chocolate chips, milk, and 6 tablespoons butter in a small saucepan over low heat.
• Heat the mixture over low heat, stirring constantly, until the chocolate and butter are fully melted and the topping is smooth and shiny. Keep the heat low so the mixture does not boil.
• Remove the saucepan from the heat and let the topping cool briefly until warm but still pourable (a few minutes).
• Pour the warm chocolate topping evenly over the cooled brownies, then immediately sprinkle the rainbow chips on top so they adhere.
• Refrigerate the frosted brownies for 1 hour to set the topping.
• Use the parchment to lift the brownies from the pan, place on a cutting board, and cut into 24 bars. Store leftovers covered in the refrigerator.