In a small bowl, whisk together all-purpose flour, baking soda, salt, cornstarch, and matcha powder.
In a mixing bowl, cream together the room temperature butter, brown sugar, and white sugar until light and fluffy, using a hand mixer or stand mixer fitted with a paddle attachment, for about 1-2 minutes.
Add the egg, egg yolk, and vanilla extract to the creamed mixture. Beat until smooth, scraping down the sides of the bowl as needed.
Gradually add the dry ingredients to the wet mixture, mixing until a dough forms.
Stir in the white chocolate chips and allow the dough to rest for 20 minutes.
Preheat the oven to 350°F and line a baking sheet with parchment paper.
Scoop out portions of the dough, about the size of a 1/4 to 1/3 measuring cup, spacing them about 6 cookies per sheet.
Bake in the preheated oven for approximately 11 to 12 minutes until the edges turn golden and the centers remain slightly soft. The cookies will continue to bake on the hot pan once removed from the oven.
Let the cookies rest on the pan for about 10 minutes or until they firm up before transferring them to a cooling rack to cool completely. Enjoy your delightful matcha white chocolate chip cookies!