Preheat your oven to 350°F (175°C) and line a 9x13 inch baking dish with parchment paper, allowing some overhang for easy removal. Lightly mist the parchment with baking spray to prevent sticking.

In a large mixing bowl, cream together the melted unsalted butter, granulated sugar, brown sugar, and vanilla extract until the mixture is smooth and well combined. Add the eggs one at a time, mixing until fully incorporated after each addition.

In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, cornstarch, and a pinch of salt. Gradually fold the dry mixture into the wet ingredients, alternating with the milk until you see a thick and luscious batter come together seamlessly.

Pour the brownie batter into the prepared baking pan, using a spatula to smooth the top into an even layer. Bake in the preheated oven for 18-25 minutes. To check for doneness, insert a toothpick into the center; it should come out with a few moist crumbs.

While the brownies cool, prepare the rich fudge ganache. In a small saucepan over medium heat, bring the heavy whipping cream to a gentle simmer. Once it bubbles slightly, remove it from the heat and add the semi-sweet chocolate chips, allowing them to rest for a few minutes.

Stir gently until the mixture is completely smooth and glossy, creating the perfect topping for your Homemade Cosmic Brownies.

Once the ganache is smooth, pour it over the cooled brownies, using a spatula to spread it evenly across the surface. Before the ganache sets, sprinkle the rainbow chips generously on top.

Allow the ganache to set at room temperature for about 30 minutes before slicing the brownies into squares.