Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper. Beat 1 cup of softened unsalted butter until creamy. Gradually add ½ cup granulated sugar and ½ cup light brown sugar until fluffy. Incorporate 1 egg yolk and 1 teaspoon vanilla extract, blending fully.

Whisk together 2 cups all-purpose flour, 2 tablespoons cornstarch, and a pinch of salt in a separate bowl. Gradually add to wet ingredients, stirring until just combined.

Scoop tablespoon-sized portions, roll into balls, and chill in the freezer for about 30 minutes. Press your thumb gently in the center of each ball to create a well for the jam filling.

Warm your desired jam slightly in the microwave and fill each indentation. Bake for approximately 11 minutes or until edges are golden brown.

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