Whisk the wet ingredients. In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth and well combined.
Add the egg and vanilla. Whisk until the mixture looks smooth and slightly glossy.
Add the dry ingredients straight in. Add the flour, baking soda, salt, and matcha powder directly into the wet mixture. Fold together with a spatula just until no dry streaks remain. Don't overmix, or you'll end up with tough cookies.
Fold in the white chocolate. Gently fold in the white chocolate chips until evenly distributed throughout the dough.
Chill the dough. Scoop the dough into balls and place on a tray or plate. Chill in the fridge for at least 1 to 2 hours, or overnight. This step is non-negotiable, it firms up the dough and keeps your cookies from spreading thin.
Bake. Preheat your oven to 180°C (350°F). Place chilled dough balls on a parchment-lined baking sheet, 2 to 3 inches apart. Bake for 11 to 14 minutes, until the edges are set and the centers still look slightly underdone.
Cool and devour. Let the cookies rest on the tray for 5 minutes before moving them to a wire rack. Or don't wait, I won't judge you for eating one warm off the tray.