1. In a small bowl, whisk together the all-purpose flour, cocoa powder, cornstarch, baking soda, and salt. Set aside.

2. In a large mixing bowl using an electric hand mixer, or use the bowl of a stand mixer with the paddle attachment, cream together the butter, light brown sugar, and sugar until smooth, fluffy, and lighter in color. At least 2-3 minutes beaten on medium-high speed.

3. Add in the egg, egg yolk, and vanilla extract. Beat until thoroughly combined.

4. Gradually add the dry ingredients (from the smaller bowl) into the wet ingredients, beating on low speed just until combined and a soft dough is formed. Gather the dough into a ball and wrap it in plastic wrap. Place the wrapped dough ball in the refrigerator to chill for at least 1 hour or up to overnight.

5. Once the dough has chilled, preheat the oven to 350℉ (175°C). Prepare two cookie sheets by lining with parchment paper or a silicone baking mat.

6. Use a 2 tablespoon cookie scoop (medium cookie scoop) to portion out the dough and roll it into a dough ball. Place 12 dough balls per cookie sheet.

7. Bake for 12-15 minutes or until the edges are firm and the centers appear just slightly underdone. Remove the cookies from the oven and allow them to cool on the cookie sheet for about 10 minutes before moving them to a wire cooling rack to finish cooling completely.

8. Once the cookies are cooled to room temperature, place the semi-sweet chocolate chips and heavy cream into a microwave-safe bowl. Heat in 20-second intervals, stirring between each, until the chocolate is completely melted and smooth. *It’s important to stir between each 20-second interval to ensure you don’t overheat the ganache. Once it’s smooth and melted, don’t heat it any longer.*

9. Gently spread the melted chocolate ganache on top of each cookie with an offset spatula or spoon. Sprinkle each cookie with some rainbow chip sprinkles. Let the ganache set and cool for about 20 minutes before enjoying a cookie.