Preheat oven to 300 F and line two cookie sheets with parchment paper or a silicone baking mat. Place oven rack in the center of the oven.

In the bowl of a stand mixer using the paddle attachment, or in a large bowl using a hand mixer, cream butter with the powdered sugar on medium speed until smooth. Add the egg yolk and vanilla and mix until well combined.

Sift the dry ingredients (flour, cornstarch, cocoa powder, and salt) into the butter mixture and mix on low speed until combined.

Place a star tip in a disposable piping bag. Use a rubber spatula to scoop the cookie dough into the bag.

Hold the piping bag perpendicularly about a half inch off the prepared baking sheet and press to pipe a star shape. Top with a chocolate chip.

If the dough is too stiff to pipe, put back in the mixer and add 1 tsp milk at a time until the dough reaches a pipe-able consistency.

Chill piped cookie dough for at least 30 minutes, or up to overnight.

Bake 15-17 minutes until just set.

Remove cookies from oven and let cool for 5 minutes on the sheet, before removing to a wire rack to cool completely.