In a large bowl, mix 2 cups all-purpose flour, 1 teaspoon baking powder, and ½ cup powdered sugar until combined.

Melt 1 cup butter and add it along with 2 tablespoons vanilla extract to the dry ingredients. Mix until a smooth dough forms.

Cover and refrigerate the dough for 30 minutes to firm up.

Wash and blend 2 cups mulberries, then transfer to a saucepan and add 1 cup sugar.

Bring to a boil over medium-high heat, then reduce heat and simmer until thickened. Add 1 tablespoon lemon juice and lemon seeds and cook until jam consistency forms.

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

Shape dough into small balls and place them on the baking sheet.

Press a thumbprint into each cookie and fill with about 1 teaspoon mulberry jam.

Bake for 12–15 minutes or until edges are lightly golden.

Cool on a rack before serving to allow the jam to set.