Method

1. In a medium bowl, whisk together the flour, salt and cornstarch until completely combined.
2. In a stand mixer or with a hand mixer in a separate large bowl, beat sugar and butter together until combined for about 2 minutes on medium low. Scrape down the bowl once within the process. Don’t overmix; you don’t want to incorporate too much air into the mixture.
3. Add the vanilla and egg yolk; mix until incorporated. Scrape down the sides of the bowl to ensure even mixing.
4. On low speed, add the dry ingredients until it comes together into a dough.
5. Take out half of the batter and set in the medium-sized bowl where your dry ingredients were. Set aside.
6. Add the matcha powder to the remaining half of the dough. Mix together until consistently green.
7. Once matcha is incorporated, split both doughs in half, giving you four total pieces of dough—two green pieces and two plain pieces.
8. Set out two pieces of plastic wrap.
9. On one of the plastic wrap pieces, stack the dough pieces on top of one another, alternating colors. Start to carefully knead the dough into one mass, creating swirls within the dough. Be careful not to overmix the colors together. You want to see the defined colors; if you mix it too much, you’ll lose the beautiful definition.
10. Separate the swirly dough in half and roll into two logs on the separate pieces of plastic wrap. Logs should be about 8 inches long.
11. Let the two dough logs cool in the fridge for at least 2 hours or overnight.
12. Once cooled, remove from the fridge. Set the oven to 325℉.
13. Pour ¼ cup of sugar onto a plate and brush the outside of the dough with the remaining egg white.
14. Roll the dough in the sugar until completely coated. You’ll want to make sure there aren’t bald spots; the cookie needs the sugar around the edge to balance the matcha.
15. Slice the dough into ½-inch slices and see how your patterns turned out. Place cookie slices onto a parchment-lined cookie sheet. Separate out, but there’s no need to leave too much space between the cookies. You should be able to fit the slices from one log onto one cookie sheet.
16. Bake for 18 minutes and let cool completely.