Cream the softened unsalted butter in a mixing bowl until smooth and pale.
Gradually add in granulated sugar and light brown sugar, beating until creamy.
Add egg yolk and vanilla extract, mixing thoroughly.
In another bowl, whisk together flour, cornstarch, and salt. Gradually add to wet ingredients.
Scoop about one tablespoon of the mixture and shape into a ball. Optionally roll in sugar.
Press your thumb into the center of each cookie ball to create an indent.
Chill the dough for about 30 minutes then preheat the oven to 375°F (190°C).
Microwave jam for about 10 seconds to soften, then fill each indent.
Bake for about 11 minutes until edges are golden.
Cool on the baking sheet for 5 minutes, then transfer to a wire rack.