In the bowl of a stand mixer or with an electric hand mixer, cream room temperature butter with sugar, baking powder, baking soda, and salt until smooth, light in color, and fluffy in texture.
Add egg yolks and beat until homogenous.
Add flour and mix until a dough forms.
Split dough in thirds.
Add matcha to one third of the dough and mix until the color is evenly green throughout.
Add black sesame paste to another third of the dough and mix again until the color is evenly dispersed. Leave one third of the dough plain (option: for my Halloween monster cookies, I added 1/8 tsp of ube extract for a purple dough instead of leaving it white).
You can eyeball this next step of swirling the cookie dough colors together, but if you want very even cookies, divide each colored dough into 10 smaller portions. Take one portion of green, one gray, and one of the white cookie doughs and roll them into a ball.
Repeat with the remaining cookie dough until you have 10 swirled cookie dough balls.
Place cookies on a lined baking sheet about two inches apart. Chill in the fridge while you preheat the oven to 350 degrees F.
When oven is ready, bake for 12 minutes or until cookies easily lift off the tray. Leftovers keep in an airtight container at room temperature.