Step 1: Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on two sides to easily lift the brownies out later. Lightly grease the exposed sides of the pan.
Step 2: In a medium saucepan, melt the unsalted butter over low heat. Once melted, remove from heat and stir in the granulated sugar and brown sugar until well combined. The mixture might look grainy, that's okay.
Step 3: Add the eggs, one at a time, beating well after each addition until thoroughly incorporated and the mixture is glossy. Stir in the vanilla extract.
Step 4: In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, salt, and baking powder. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix, as this can lead to tough brownies. Gently fold in the dark chocolate chips.
Step 5: Pour the brownie batter into the prepared baking pan, spreading it evenly with a spatula. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs, not wet batter. The edges should be set, but the center should still look a little fudgy. Do not overbake!
Step 6: Let the brownies cool completely in the pan on a wire rack before frosting. This is essential for clean cuts and preventing the frosting from melting.
Step 7: For the fudge frosting, in a medium saucepan, melt the butter over low heat. Once melted, stir in the unsweetened cocoa powder until smooth.
Step 8: Remove from heat and gradually whisk in the powdered sugar, milk, and vanilla extract until smooth and creamy. If the frosting is too thick, add more milk a teaspoon at a time until it reaches a spreadable consistency. If it's too thin, add a bit more powdered sugar.
Step 9: Once the brownies are completely cool, spread the fudge frosting evenly over the top. Immediately scatter the rainbow chip sprinkles generously over the frosting before it sets.
Step 10: Allow the frosting to set for at least 30 minutes at room temperature, or in the refrigerator for 15-20 minutes, before slicing. This helps the sprinkles adhere and makes for cleaner slices. Store any leftover cosmic brownie recipe portions in an airtight container.