Heat the oven to 325°F (160°C) and line a large baking sheet with parchment paper.
In a large bowl, stir the butter and powdered sugar until smooth and creamy. Mix in the vanilla and salt.
Add the flour and cornstarch and stir gently until a soft dough forms.
Cover the bowl and chill the dough for 20 minutes.
Scoop about 1 tbsp of dough, roll into a ball, and place on the baking sheet 2 inches (5 cm) apart.
Press a deep dent in each ball with your thumb, then fill each with about 1/2 tsp of raspberry jam.
Bake for 16 to 18 minutes, until the edges are light golden.
Cool on the pan for 10 minutes, then move to a wire rack. Dust with powdered sugar if you like.