Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper. Cream together 1/2 cup butter, 1/4 cup granulated sugar, and 1/4 cup brown sugar until fluffy.

Add 1 egg yolk and 1 teaspoon vanilla extract, mixing until fully incorporated.

Whisk together 1 cup flour, 2 tablespoons cornstarch, and a pinch of salt in a separate bowl. Gradually mix into wet ingredients until crumbly dough forms.

Scoop tablespoon-sized dough portions and roll into balls. Indent centers with your thumb; chill in fridge for 30 minutes.

Warm desired jam slightly. Fill each indentation with a small dollop of jam after chilling.

Bake for 11 minutes or until edges are lightly golden. Let cool for 5 minutes before transferring to a wire rack.

Enjoy your homemade Thumbprint Cookies!