Prepare the Shortbread Dough:

1. In a large mixing bowl, cream together butter and sugar until light and fluffy.
2. Mix in vanilla extract until well combined.
3. In a separate bowl, whisk together flour and salt.
4. Gradually add the dry ingredients to the butter mixture, mixing until a soft dough forms.
5. Shape the dough into a disk, wrap in plastic wrap, and refrigerate for at least 1 hour until firm.

2. Prepare the Raspberry Filling:

1. In a small saucepan, combine raspberry jam and lemon juice.
2. Warm over low heat, stirring occasionally, until smooth and slightly thinned.
3. Remove from heat and cool to room temperature.

3. Assemble the Tarts:

1. Preheat oven to 350°F (175°C).
2. On a lightly floured surface, roll out the chilled dough to about ¼-inch thickness.
3. Using a round cutter slightly larger than your tart molds, cut out circles of dough.
4. Gently press each dough circle into the tart molds, ensuring an even thickness along the bottom and sides.
5. Spoon 1-2 teaspoons of raspberry filling into each tart shell.

4. Bake:

1. Place filled tart molds on a baking sheet.
2. Bake for 15-20 minutes, or until the edges are lightly golden.
3. Remove from the oven and cool in molds for 10 minutes.
4. Transfer to a wire rack to cool completely.

5. Serve:

1. Once cooled, dust with powdered sugar if desired.
2. Enjoy with a cup of tea or coffee.