Directions:

• Step 1: Preheat your oven to 350°F (175°C).
• Step 2: Line your 9x13 inch baking pan with parchment paper.
• Step 3: In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until well combined. Add the eggs and egg yolk one at a time, mixing well after each addition. Stir in the vanilla extract and vegetable oil until smooth.
• Step 4: In another bowl, whisk together the flour, cornstarch, cocoa powder, salt, and baking soda.
• Step 5: Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
• Step 6: Fold in the semi-sweet chocolate chips gently until evenly distributed.
• Step 7: Pour the brownie batter into the prepared baking pan and spread it evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
• Step 8: While the brownies are baking, melt the 6 tbsp of butter for the frosting in a small saucepan over low heat. Stir in the milk and cocoa powder until smooth.
• Step 9: Once the brownies are done, let them cool for about 10 minutes, then pour the frosting over the warm brownies and spread it evenly. Sprinkle the rainbow chips on top.
• Step 10: Allow the brownies to cool completely, then lift them out using the parchment paper and slice into squares.