Preheat your oven to 350°F (175°C). Line two baking sheets with parchment or a silicone mat—this prevents sticking and browning on the bottom.

In a large bowl, beat the softened butter with granulated and brown sugar until light and fluffy, about 2–3 minutes. You’re aiming for a pale, airy mixture—this creates tender cookies.

Crack in the egg, then pour in vanilla. Beat another 30 seconds until fully combined. Scrape down the sides so nothing’s hiding up top.

In a separate bowl, whisk flour, baking powder, baking soda, and salt. Gradually add to the butter mixture and stir until just combined—overmixing can yield tough cookies.

Toss blueberries, tapioca starch, sugar, and lemon zest in a small bowl. The fruit should be evenly coated—this ensures a jam-like consistency once baked.

Scoop 1½-tbsp portions of dough onto your counter. Flatten slightly into 2-inch rounds. Place about 1 tsp of the blueberry filling in the center of half your rounds. Top with another dough round, sealing edges by pinching lightly—think little dough pockets.

Transfer cookies to your prepared sheets, spacing 2 inches apart. Bake 12–14 minutes, until edges are set and tops show a hint of golden. You’ll smell that sweet blueberry aroma when they’re ready.

Let cookies rest 5 minutes on the baking sheet, then slide onto a wire rack to cool fully. Warm filling is luscious, but these taste lovely at room temperature, too.