1. Prepare the Crust:

1. Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
2. In a large bowl, cream together butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in vanilla.
3. In a separate bowl, whisk together flour, baking powder, and salt. Gradually add to the butter mixture, mixing until just combined.
4. In a small bowl, mix blueberry jam and lemon juice until smooth. Drop spoonfuls of jam onto the cookie dough and gently swirl with a knife.
5. Drop tablespoon-sized portions of dough onto prepared baking sheets, spacing them 2 inches apart. Bake for 10-12 minutes or until edges are lightly golden.
6. Let cookies cool on baking sheets for 5 minutes before transferring to a wire rack to cool completely.