1-First step: prep the pan and oven Preheat your oven to 350°F. Line an 8×8 inch metal baking pan with parchment paper or foil, then spray it with cooking spray. A light-colored metal pan works best because it helps the brownies bake evenly without overbrowning the edges.

2-Second step: melt the butter Place 10 tablespoons of unsalted butter in a large microwave-safe bowl and melt it on high for about 90 seconds. Stir it well so it turns fully liquid and smooth. Using butter instead of oil gives this cosmic brownies recipe a richer flavor and a fudgier texture.

3-Third step: mix the sugars and cocoa Stir in the granulated sugar, light brown sugar, and sifted cocoa powder. Mix until everything looks thick and glossy. Let the mixture cool slightly before adding the eggs so they do not scramble.

4-Fourth step: add the wet ingredients Mix in the vanilla extract, eggs, egg yolk, and corn syrup. Stir until the batter looks smooth and well blended. The extra egg yolk adds richness, while the corn syrup helps the brownies stay chewy and soft.

5-Fifth step: add the dry ingredients Add the flour, cornstarch, and fine sea salt. Stir just until combined. Do not overmix, or the brownies can turn dense instead of fudgy. If you are using a digital scale, this is the best time to confirm your measurements are accurate.

6-Sixth step: bake the brownies Pour the thick batter into the prepared pan and spread it evenly to the corners. Bake for 25 minutes at 350°F. The center should look set but still soft. A toothpick may come out with a few moist crumbs, which is exactly what you want for a fudgy brownie.

7-Seventh step: cool completely Let the brownies cool fully in the pan. This matters because warm brownies can make the frosting slide off and can lead to messy cuts. If you are short on time, you can place the pan on a wire rack to help it cool faster.

8-Eighth step: make the chocolate topping Warm the heavy cream in the microwave for 1 minute. Add the semisweet chocolate chips and let them sit for 3 minutes. Then stir until the mixture becomes smooth and shiny. This quick ganache gives the brownies their classic cosmic brownie look.

9-Ninth step: frost and top Pour the ganache over the cooled brownies and spread it evenly with a spatula. Sprinkle on rainbow candy-coated chocolate chips, rainbow sprinkles, or another candy topping. If you want the nostalgic store-bought look, sprinkle the topping while the ganache is still soft so it sticks well.

10-Tenth step: chill and slice Refrigerate the brownies for 1 to 2 hours until the topping sets. Then cut them with a large sharp knife warmed under hot water, wiping the blade between slices. This gives you clean squares with neat edges.