Double Chocolate Butter Cookies are crisp, buttery bittersweet cookies with melty chocolate chips in the middle. They look fancy but are so easy to make!
Preheat oven to 300℉/150℃ and line a baking sheet with parchment paper or silicone baking mat.
Using a mixer or spatula, mix butter and confectioners sugar until well incorporated. Add egg yolk and vanilla extract. Mix until well combined.
Sift in cake flour, cornstarch, cocoa powder, and salt. Mix with a spatula until well incorporated.
Add a French star piping tip to the piping bag. Spoon a little bit of dough into the piping bag and try to pipe into star shapes about 1-inch/2.5 cm in diameter on the prepared baking sheet. If the dough is too thick to pipe, transfer the dough back to the mixing bowl and add 1 tsp of milk. If the dough is creamy enough to pipe, continue piping the dough on the baking sheet.
Place a chocolate chip/callet in the middle of each cookie.
Bake the cookies for 18-20 minutes.
Remove from the oven and immediately transfer them to a wire rack to cool completely.
Notes
Make sure to measure the flour correctly if using volume measurement. The cookies will end up too hard to pipe if you add too much flour. Use a spoon to scoop the flour into the measuring cup. Then, level the top with a knife or other straight-edged utensil.