Preheat the oven to 350 degrees Fahrenheit. Prepare a 9x11 baking sheet with parchment paper and set aside.
In a mixing bowl, cream the butter, brown sugar, and granulated sugar together until creamy and fluffy.
Add the egg, corn syrup, and vanilla extract, bending again with a hand mixer until combined.
Continue by adding the dry ingredients: flour, cocoa powder, baking soda, and salt. Mix on medium speed until all the wet and dry ingredients are incorporated.
Use a cookie scooper, to take a large ¼ cup size of dough into your hands and roll it into a ball. Place the dough ball on the baking sheet and flatten with your hand to about ½ inch thick.
Repeat this process with the rest of the cookie dough.
Bake the cookies for 11 minutes. Do not overbake. If they look a little undercooked, they will set as they cool.
While the cookies bake, make the ganache. Heat the heavy cream in a small saucepan until warm and steaming, but not boiling.
Pour the hot cream over the chocolate chips and let sit for 3 minutes before touching. After 3 minutes have passed mix until shiny and smooth.
Place a tablespoonful of ganache in the center of each brownie cookie and use a circular motion with the spoon to spread it towards the edge. Top with rainbow chocolate sprinkles.
Place the decorated cookies in the fridge in a single layer and let the ganache harden for 10 minutes or at room temperature for an hour.