Step 1: Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper or silicone baking mats.
Step 2: In a medium bowl, whisk together the all-purpose flour, sieved matcha powder, baking soda, and salt until well combined and the color is uniform.
Step 3: In a large bowl using a hand mixer or a stand mixer, cream together the softened butter, granulated sugar, and brown sugar on medium-high speed for about 2-3 minutes until light and fluffy.
Step 4: Add the egg and vanilla extract to the butter mixture, beating well and scraping down the sides of the bowl as needed.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed just until the flour streaks disappear. Do not overmix.
Step 6: Fold in the white chocolate chips by hand using a spatula.
Step 7: Scoop about 1.5 tablespoons of dough for each cookie and place them on the prepared baking sheets, spaced 2 inches apart.
Step 8: Bake for 10-12 minutes, or until the edges are just set but the centers still look slightly soft. Do not overbake, or they will lose their green color.
Step 9: Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.