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.