Cream the butter and sugars by beating 1 cup of softened unsalted butter with ½ cup granulated sugar and ½ cup light brown sugar until well-aerated and fluffy.

Incorporate the egg yolk and vanilla extract by adding 1 egg yolk and 1 teaspoon of vanilla extract to the butter mixture and beating until smooth.

Combine the dry ingredients by whisking together 2 cups all-purpose flour, 2 tablespoons cornstarch, and a pinch of salt, then add to the wet ingredients.

Form dough balls by rolling tablespoon-sized portions of dough into smooth balls and place on a parchment-lined baking sheet.

Create the thumbprint indents by pressing your thumb or back of a spoon into the center of each dough ball.

Chill the dough by placing the baking sheet in the freezer for 30 minutes.

Preheat the oven to 375°F (190°C) while the dough chills.

Fill with jam by using a small spoon to add approximately 1 teaspoon of jam to each indentation.

Bake the cookies for about 11 minutes until the edges are lightly golden.

Cool before serving by transferring baked cookies to a wire rack after a few minutes.