Preheat the oven to 350°F (175°C) and line a large baking sheet with parchment paper.

In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until the mixture is well combined.

Add the egg and vanilla extract, whisking vigorously for 1 minute until the batter is smooth and glossy.

Fold in the sifted cocoa powder, flour, baking soda, and salt using a spatula until no dry streaks remain.

Using a cookie scoop, portion approximately 2 tablespoons of dough for each cookie and place on the prepared baking sheet.

Bake for 9 to 11 minutes until the edges are firm but the centers remain soft.

Allow the cookies to cool completely on the baking sheet to ensure they set properly.

Prepare the ganache by heating heavy cream until simmering, then pour it over the semi-sweet chocolate chips.

Let sit for 2 minutes, then stir until a smooth, glossy ganache forms.

Spoon a tablespoon of ganache onto the center of each cooled cookie and spread it toward the edges.

Immediately sprinkle the rainbow candy-coated chocolate chips over the wet ganache and let set for 20 minutes.