Preheat the oven to 375 degrees F. Prepare a baking sheet by lining with parchment paper.
Combine the butter, cream cheese, and sugar into a large mixing bowl. Beat until a smooth consistency.
Add in the egg yolk, vanilla, and food colouring. Mix again until combined.
In a separate bowl combine the flour, cocoa, and corn starch. Mix to combine and then add to the creamy mixture.
Mix this until the dough creates a ball and comes away from the sides of the bowl. If the dough is too sticky, add in a little more flour and continue mixing.
Roll out the dough to 1/4" thickness evenly between two sheets of parchment paper.
Remove the top parchment, and cut shapes with a medium sized heart cookie cutter.
Transfer the heart cookie shapes to the prepared baking sheet.
Bake for 14 to 16 minutes. Let sit for a few minutes to cool on baking sheet and then transfer to a cooling rack. NOTE: if you roll out thinner, I would reduce to baking time.
Cool overnight before frosting with royal icing. Details on decorating is below in the blog post.