In a small saucepan, combine blueberries and lemon juice. Simmer over medium heat for 10-15 minutes, crushing berries with a spoon, until reduced to a thick jam (about 1/3 to 1/2 cup). Let cool completely.

In a large mixing bowl, combine 1/2 cup of the granulated sugar with the lemon zest. Rub together with fingers until fragrant.

Add the softened butter and beat on medium-high speed until light and fluffy.

Beat in the egg and vanilla extract, followed by the cooled blueberry reduction. The mixture will be bright purple.

Whisk together flour, baking powder, and salt. Gradually add to the wet ingredients, mixing until just combined. The dough will be sticky.

Cover and refrigerate the dough for at least 2 hours (this is mandatory).

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

Place remaining 1/2 cup granulated sugar in one bowl and powdered sugar in another. Scoop dough into balls.

Roll each ball first in granulated sugar, then heavily in powdered sugar.

Bake for 10-12 minutes until puffed and cracked. Let cool on the pan for 5 minutes before transferring to a rack.