In a heatproof bowl over a pot of simmering water, melt the butter and semi-sweet chocolate chips together until smooth. Remove from heat and let it cool slightly.

In a separate bowl, whisk together the granulated sugar, brown sugar, eggs, and vanilla extract until smooth and slightly thickened. Add the chocolate mixture, then whisk until combined.

In a separate bowl, mix together the flour, cocoa powder, cornstarch, and salt. Add the dry ingredients to the chocolate mixture and gently fold until just combined. Avoid overmixing.

Cover the dough and refrigerate for at least 30 minutes. This helps the cookies hold their shape and creates a fudgy brownie texture.

Scoop the dough onto a parchment-lined baking sheet. Bake at 350°F for 10–12 minutes, until the edges are set but the centers are still soft.

While the cookies cool, combine the chocolate chips, heavy cream, and butter in a microwave-safe bowl. Microwave in 15–20 second intervals, stirring between each interval, until smooth and fully melted.

Let the ganache cool slightly, then spread it over the tops of the cookies. Sprinkle with rainbow candy-coated chocolate chips.

Let the topping set before serving for the best Cosmic Brownie experience.