Double Chocolate Butter Cookies are crisp, buttery bittersweet cookies with chocolate callets/chips in the middle. They look fancy but are so easy to make!
Preheat oven to 340℉/170℃ and line two baking sheets with parchment paper.
Using a hand mixer or stand mixer fitted with a paddle attachment, cream together butter and confectioners' sugar on high speed until pale and fluffy.
Add egg yolk and vanilla. Beat the mixture until well combined.
Sift in cake flour, cocoa powder, cornstarch, and salt. Beat on low speed until just combined. Don't overmix.
Add a large French star piping tip to the piping bag. Transfer the dough into the piping bag and try piping 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 into swirls, leaving 2 inches apart on the baking sheet.
Place a chocolate callet/chip in the middle of each cookie.
Chill in the fridge for 30 minutes to let the dough firm up so that they keep their shape better during baking. This step is optional.
Bake cookies for 15-18 minutes or until the edges are light brown. Bake for 15 minutes for softer cookies, bake for 18 minutes for crisper cookies.
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 dry if you add too much flour.