Begin by placing 1 cup of softened unsalted butter into a mixing bowl. Beat until creamy, about 2-3 minutes.

Gradually add ½ cup granulated sugar and ½ cup light brown sugar, mixing until fluffy and light in color.

Crack in one large egg yolk and pour in 1 teaspoon of vanilla extract, mixing until smooth.

In a separate bowl, whisk together 2 cups of all-purpose flour, 2 tablespoons of cornstarch, and ½ teaspoon of salt.

Slowly incorporate the dry ingredients into the wet mixture until a slightly sticky dough forms.

Scoop out tablespoon-sized portions of the dough and roll into smooth balls. Optionally roll in granulated sugar.

Press a thumb into the center of each dough ball to create an indentation, then chill dough balls in the freezer for 30 minutes.

Preheat your oven to 375°F (190°C). Warm your choice of jam and fill each indentation carefully.

Bake cookies for about 11-13 minutes until edges are golden but centers remain soft.

Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.