In a small saucepan over medium heat, add the blueberries and cook for 5–7 minutes, stirring often, until the berries burst and become jammy. Remove from heat and cool slightly, then blend or mash until smooth. You should have about 1/2 cup of puree.

In a large bowl, beat the softened butter, granulated sugar, and brown sugar with an electric mixer until light and fluffy, about 2–3 minutes.

Beat in the egg, vanilla extract, and the cooled blueberry puree. The mixture may look slightly separated at first, but it will come together once you add the dry ingredients.

In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry mixture to the wet ingredients, mixing until just combined. The dough will be soft and slightly sticky.

Cover the bowl and chill the dough in the refrigerator for at least 1 hour. Chilling helps the cookies hold their shape and allows the flavors to meld.

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

Scoop about 1 1/2 tablespoons of dough and roll into balls. Roll each dough ball generously in powdered sugar, coating well on all sides.

Place the coated dough balls on the prepared baking sheets, spacing about 2 inches apart. Bake for 10–12 minutes, or until the cookies have spread slightly and developed a crinkly top. The centers should look soft but not wet.

Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack. Enjoy warm or at room temperature.