Preheat your oven: Set it to 350°F (175°C) and line a baking sheet with parchment paper to prevent sticking.
Cream the butter and sugars: In a mixing bowl, use 1/2 cup softened unsalted butter, 1/2 cup granulated sugar, and 1/2 cup brown sugar. Mix them until they are fluffy and light in color, reflecting an airy texture.
Add the egg and vanilla: Crack in 1 large egg and pour in 1 tsp of vanilla extract. Mix until fully combined, ensuring a homogenous batter.
Whisk the dry ingredients: In a separate bowl, whisk together 1 1/4 cups all-purpose flour, 1/2 cup cocoa powder, 1/2 tsp baking soda, and 1/4 tsp salt. This will allow for even distribution of the leavening agent and cocoa powder.
Combine wet and dry ingredients: Gradually add the dry mixture to the creamed mixture, stirring until just combined. Then, mix in 1 tbsp of milk. The dough should come together nicely. Lastly, fold in 1/2 cup mini chocolate chips for extra chocolatey goodness.
Form the cookies: With clean hands, roll the dough into balls about 1 inch in diameter. Place them on the prepared baking sheet, spaced apart, and flatten them slightly with the palm of your hand.
Bake the cookies: Place the baking sheet in the oven and bake for 10-12 minutes. You're looking for edges that are set but centers that still feel soft to the touch—this will ensure a chewy cookie.
Cool the cookies: Once baking is complete, allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
Prepare the ganache: In a small saucepan, melt chocolate chips with heavy cream until smooth and decadent. Take the time to stir until there are no lumps and it’s silky.
Assemble the cosmic treats: Once cooled, drizzle the chocolate ganache over the cookies. Top with mini rainbow candy-coated chocolates and, if desired, sprinkle with white sprinkles for that extra cosmic effect.
Let the ganache set: Allow the cookies to sit until the ganache has firmed up slightly, ensuring that the toppings stay in place.