Preheat the oven to 350F degrees and line 2 cookie sheets with parchment paper or silicone baking mats.
In a large bowl beat together the butter, white sugar and brown sugar using an electric mixer.
Then beat in the egg, vanilla extract, vinegar and food coloring.
In a medium bowl whisk together the flour, cocoa powder, cornstarch, baking soda and salt.
With the mixer on low speed, beat the flour mixture into the butter mixture about 1/2 at a time. The cookie dough will be thick and sticky.
Form the dough into balls about 1 to 1.5 tablespoons in size. Using a cookie scoop makes this much easier.
Place the balls 2 inches apart on a lined cookie sheet and bake for 8-10 minutes, or until the tops are just set.
Allow to cool fully.
In a small microwave safe bowl, melt the candy melts in short 30 second bursts stirring between each.
Spread half of each cookie with the candy melts and decorate with sprinkles.
Storage: Keep cookies in an airtight container at room temperature and they will remain fresh for up to 3-4 days.Freezing: Place cookies in a resealable plastic freezer bag or container and freeze for up to 3 months.