In a saucepan, combine chopped rhubarb, sugar, and lemon juice. Cook over medium heat until rhubarb is soft and mixture thickens, about 10 minutes.

In a mixing bowl, cream together softened butter, granulated sugar, and brown sugar until light and fluffy. Stir in vanilla.

Gradually add flour, salt, and baking powder, mixing until combined.

Chill the dough in the refrigerator for 15 minutes.

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

Roll the chilled dough into balls and place them on the prepared baking sheet. Make an indentation in the center of each ball with your thumb.

Fill each indentation with the rhubarb filling.

Bake for 12-15 minutes or until the edges are lightly golden. Let cool on a wire rack.