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 and sugar until light and fluffy.

Add egg yolk and vanilla extract, mixing until combined.

Gradually add flour and salt, mixing until a dough forms.

Preheat the oven to 350?F (175?C).

Scoop tablespoon-sized portions of dough and roll into balls. Place on a baking sheet.

Use your thumb to create an indentation in the center of each ball.

Fill each indentation with the rhubarb filling.

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