Combine the flour, cornstarch, baking powder and baking soda, salt, coffee granules, cocoa powder and chocolate chips all together in a bowl and set aside.
Preheat the oven to 350F and prep a baking sheet by lining it with parchment paper and spraying it with cooking spray.
Cream the cold butter for a few minutes until softened and sticking to sides of the bowl. On medium-low, gradually add in the sugars. Then, add in the eggs and mix to combine.
Turn the mixer off and dump in the dry ingredients all at once then turn on to the lowest setting before bumping back up to medium-low. Mix for 30 seconds or until just combined and flour streaks are still visible.
Add in the milk and combine for another few seconds until no more flour shows and the mixture resembles cookie dough. Don't overmix!
Use a large ice cream scoop or two spoons to scoop out the dough onto your prepared baking sheet.
Bake for 10 minutes and remove from the oven. Tap the baking sheet gently but firmly a few times on the counter once it's out of the oven. Then let the cookies cook on the pan out of the oven for a couple more minutes before moving to a wire rack to cool.
Microwave or heat up the heavy cream. Then pour over the chopped chocolate in a medium-sized bowl. Let the mixture sit for five minutes before stirring well. Continue stirring until the mixture resembles a shiny chocolate sauce. The more you mix, the shinier it will be. Once you have the ganache done, set it aside at room temp to cool slightly.
Once the ganache is slightly cooled, spoon over a bit over the cookie and swirl with an offset spatula or the back of a spoon. Sprinkle a few rainbow chips over the top. Allow ganache to set for another 15 minutes or so and then enjoy!
