Preheat the oven to 350°F. Line an 8×8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.

In a saucepan, melt the butter and chopped chocolate together. Stir well and allow the mixture to cool slightly until warm but not hot.

In a large mixing bowl, whisk together the granulated sugar, brown sugar, eggs, egg yolk, and vanilla extract. Whisk vigorously for 1–2 full minutes, until the mixture becomes pale, thick, and glossy.

Slowly pour the melted chocolate mixture into the egg mixture while whisking continuously, ensuring it blends smoothly and evenly.

Using a rubber spatula, gently fold in the flour, cocoa powder, cornstarch, and salt just until combined and no dry streaks remain. Be careful not to overmix.

Spread the batter evenly into the prepared pan. Bake for 20–23 minutes, until the edges are set but the center still appears slightly underbaked. A toothpick inserted should come out with thick, moist crumbs rather than clean.

Allow the brownies to cool completely in the pan, then transfer to the refrigerator and chill for at least 1 hour before topping.

Place the chopped chocolate and honey in a heat-safe bowl. Heat the heavy cream until just beginning to simmer, then pour it over the chocolate. Let sit for 1 minute, then stir until completely smooth and glossy.

Pour the ganache over the chilled brownies and spread into an even layer. Immediately sprinkle the rainbow candy-coated chocolate chips or sprinkles evenly over the top.

Return the brownies to the refrigerator and chill for 45–60 minutes, or until the ganache is set but still soft.

Lift the brownies out of the pan using the parchment overhang. For clean cuts, use a sharp knife wiped clean between slices. Cut into small rectangles for a classic cosmic brownie look.

Store brownies refrigerated for the best texture.