1. Preheat the oven to 350°F.
2. In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar with an electric mixer on medium-high speed for about 2 minutes, until the mixture looks pale and fluffy.
3. Add the egg, vanilla, and light corn syrup. Mix on low until everything is fully combined, scraping down the bowl as needed.
4. Sprinkle in the baking soda and salt, then mix briefly until incorporated.
5. Add the cocoa powder and flour. Mix on medium speed just until the dough comes together.
6. Scoop the dough into 1/4-cup portions, roll each portion into a ball, and gently press each one into a thick disc about 1/2 inch thick.
7. Bake for 11 to 12 minutes. Do not overbake them; the centers may look slightly soft, but they will finish setting as they cool on the pan.
8. Let the cookies cool completely before adding the topping.

To Make the Chocolate Ganache

1. Place the chocolate chips in a medium heat-safe bowl.
2. Warm the heavy cream in the microwave for 30 to 45 seconds, or until it begins to bubble. Use a bowl or measuring cup with plenty of extra room so it does not overflow.
3. Pour the hot cream over the chocolate chips. Do not stir right away. Let it sit for about 3 minutes.
4. Stir slowly with a fork until the mixture becomes smooth and glossy. If needed, microwave in 10-second bursts at 50% power, stirring between each burst, until fully melted.

To Decorate

1. Spoon about 1 tablespoon of ganache onto each cooled cookie.
2. Finish with rainbow chip sprinkles.
3. Refrigerate for 30 to 60 minutes so the ganache firms up.