In a medium saucepan, combine fresh or frozen blueberries with lemon juice over medium heat. Stir frequently and allow the mixture to simmer for 10-15 minutes until it thickens to a jam-like consistency. Remove from heat and let cool completely.

In a large mixing bowl, combine granulated sugar and lemon zest. Rub the zest into the sugar until fragrant.

Add softened unsalted butter to the lemon-sugar mixture and beat until light and fluffy. Add egg and vanilla extract, mixing until incorporated. Gradually stir in the cooled blueberry reduction.

In a separate bowl, whisk together flour, baking powder, and salt. Gradually add dry mixture to wet ingredients until just combined.

Cover the mixing bowl with plastic wrap and refrigerate the dough for at least 2 hours.

Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.

Once the dough is chilled, scoop and roll into balls. Roll each ball in granulated sugar and then dust with powdered sugar.

Arrange dough balls on baking sheets, spacing them 2 inches apart. Bake for 10-12 minutes until cookies puff and form cracks on the surface. Cool on sheets for 5 minutes before transferring to a wire rack.