Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper and set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream the softened butter and brown sugar on medium-high speed until light and fluffy, approximately 2 minutes.

Scrape down the sides and bottom of the bowl. Add the egg and vanilla extract, then beat on medium-high speed until the mixture is pale and well incorporated, about 2 minutes.

Reduce the mixer speed to low. Add the flour, cocoa powder, baking soda, and salt. Mix just until combined and no dry streaks remain.

Using a 1½-ounce cookie scoop (approximately 3 tablespoons), portion the dough into evenly sized balls. Roll gently between your hands and place on the prepared baking sheets, spacing cookies about 2 inches apart.

Bake for 11–12 minutes, or until the cookies are set around the edges but still slightly soft in the center.

Remove from the oven and allow cookies to rest on the baking sheets for 5 minutes. Transfer to a wire rack and cool completely before adding ganache.

To prepare the ganache, heat the heavy cream in a microwave-safe bowl until hot but not boiling, about 30–45 seconds. Add the chocolate chips and let stand for 1 minute.

Stir until the mixture is smooth and glossy.

Spoon approximately 1 tablespoon of ganache onto each cooled cookie and spread evenly over the top. Sprinkle with rainbow candy-coated chocolate chips.

Allow the ganache to set at room temperature for approximately 30 minutes before serving.