Prepare Blueberry Powder: Place freeze-dried blueberries in a blender or spice grinder. Pulse until a fine powder forms, about 30 seconds. Set aside.

Cream Butter and Sugars: In a stand mixer, beat unsalted butter with granulated sugar and light brown sugar on medium speed for about 3-5 minutes until light and fluffy. Add egg, vanilla extract, and lemon zest, mixing until well incorporated.

Mix Dry Ingredients: In a separate bowl, whisk together all-purpose flour, baking powder, baking soda, and kosher salt. Gradually add dry ingredients to the wet mixture in the stand mixer, mixing until just combined.

Divide Dough: Turn the dough out onto a clean surface and divide in half. Add prepared blueberry powder to one half and mix it in thoroughly.

Shape Cookies: Roll small portions of both doughs into balls. Combine one ball of regular dough with one ball of blueberry-infused dough, folding together to create a marbled effect. Dust with granulated sugar.

Prepare for Baking: Preheat oven to 350°F (175°C). Line a baking tray with parchment paper. Arrange cookie dough balls, spacing them about 2 inches apart. Flatten slightly.

Bake: Bake for approximately 8-10 minutes, until edges are golden and centers are set but soft. Cool on the tray for a few minutes before transferring to wire racks.