Preheat oven to 340℉/170℃ and line a baking sheet with parchment paper or silicone baking mat.
Using a mixer or spatula, mix butter and confectioners sugar until combined. Add egg yolk and vanilla extract. Mix until well combined.
Sift in cake flour, cornstarch, matcha powder, and salt. Mix with a spatula until well incorporated.
Add your star piping tip to the piping bag. Spoon a little bit of 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. (*see video for a visual guide)
Bake cookies for 15-17 minutes or until the edges are light brown.
Remove from the oven and immediately transfer them to a wire rack to cool completely.
Notes
This recipe will make crisp and tender cookies with a snap. If you prefer chewier cookies, substitute cake flour and cornstarch with the same amount of all-purpose flour.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.