Preheat the oven to 350 degrees. Line 2 baking sheets with parchment paper.
Stencil heart shapes all over the parchment paper, leaving about an inch of space between each. Flip parchment paper over so that the side with pencil marks will not be the side that the food touches*
In a large mixing bowl, use an electric mixer to cream the butter and sugars together for at least 3 minutes on medium high speed until it is light and fluffy. Then, beat in the eggs until smooth. Next, add the buttermilk and food coloring and mix only until well combined, do not overmix.
In a separate bowl, whisk together the flour, cocoa, baking soda, and salt. Once combined, add these ingredients to the wet ingredients. Mix together on low speed until mixed. Add in vanilla and vinegar, mixing just until combined. The batter will look more like cake batter than cookie batter.
Place the batter into a piping bag with a Wilton 1A round tip. If you don’t have one, you could cut the corner out of a large zip-lock bag and this will work as well.
Pipe the batter into heart shapes leaving about two inches of space between them. The cookies will spread while baking.
Bake the cookies for 10-12 minutes until completely set up. Allow them to cool slightly on the pan, then move them to a wire rack to cool completely.