In a medium saucepan, combine fresh blueberries and lemon juice over medium heat. Simmer for 10–15 minutes until thickened. Remove from heat, cool completely.

In a mixing bowl, combine granulated sugar with lemon zest. Rub together until fragrant. Set aside.

In a large bowl, beat butter and lemon sugar until light and fluffy, about 2–3 minutes.

Add egg, vanilla extract, and cooled blueberry reduction to the creamed mixture. Mix on low speed until just combined.

Whisk together flour, baking powder, and salt. Gradually add to the wet mixture, mixing gently until incorporated.

Cover the bowl and refrigerate the dough for at least 2 hours, or overnight if possible.

Preheat oven to 350°F (175°C). Roll chilled dough into balls, first in granulated sugar and then in powdered sugar.

Bake for 10-12 minutes, until edges are set and centers are soft. Cool on baking sheet for 5 minutes before transferring to a wire rack.