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

Cream the Butter and Sugars: In a large mixing bowl, beat the softened butter with an electric mixer until smooth. Add the granulated sugar and powdered sugar, mixing until well combined and fluffy. Stir in the vanilla extract.

Combine Dry Ingredients: In a separate bowl, whisk together the all-purpose flour, cornstarch, and salt.

Mix Together: Gradually add the dry ingredients to the butter mixture, mixing until just combined. The dough should be soft but not sticky.

Shape the Cookies: Scoop tablespoon-sized portions of dough and roll them into balls. Place them on the prepared baking sheet, spacing them about 2 inches apart.

Make the Thumbprint: Using your thumb or the back of a teaspoon, gently press down in the center of each cookie to create a small well.

Add Jam: Fill each thumbprint with about 1/2 teaspoon of raspberry jam.

Bake: Bake in the preheated oven for 15-18 minutes, or until the edges are lightly golden. The centers may look slightly soft but will firm up as they cool.

Cool: Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.