Cream the Butter and Sugar: In a stand mixer fitted with a paddle attachment (or by hand), beat the softened butter and granulated sugar on medium-high speed until light and fluffy, about 3 minutes.
Add Wet Ingredients: Add the sourdough starter, egg, and vanilla extract. Beat until well combined.
Incorporate Dry Ingredients: Slowly beat in the flour, matcha powder, baking powder, baking soda, and salt until just combined. Fold in the chopped white chocolate.
Ferment the Dough: Cover the dough tightly and ferment at room temperature for at least 8 hours or overnight. You can refrigerate the dough after 8 hours but before baking just bring it to room temperature for a few hours 🙂
Preheat and Shape: Preheat your oven to 350°F (177°C). Roll the dough into 12-14 cookies, about 60g each. Place the cookie dough balls onto parchment-lined baking sheets and press the middles down slightly.
Bake the Cookies: Bake for 8-12 minutes or until the edges are lightly golden. Remove from the oven and let them cool completely on the baking sheet before serving.
Storage: Store the cookies in an airtight container at room temperature for up to 3 days, or in the refrigerator for up to 6 days. To reheat, bake in a 350°F (177°C) oven for 5 minutes.