Cream the butter in a large mixing bowl with an electric mixer until smooth. Make sure there are no lumps in the butter. Mix in the sugar until light and fluffy.

Add the egg yolks and vanilla extract, and mix until combined. Then, mix in half of the flour, the cornstarch, and the salt. Mix until the flour is incorporated before adding the remaining half of the flour. Continue mixing until the dough looks crumbly but feels smooth to the touch. It shouldn’t be sticky, nor should it be dry.

Use a medium (1 ½ tablespoons) cookie scoop to scoop out level scoops of dough, and roll the dough in your hands until round, smooth balls are formed. Roll the balls of dough in the extra granulated sugar, if desired.

Place the dough balls about 1 inch apart on a small, parchment-lined baking sheet or cutting board. Use the back of a ½ teaspoon measuring spoon dipped in flour to press round indentations on top of each ball of dough, pressing carefully so you don’t get too many large cracks in the dough.

If you are opening a new jar of jam or preserves or if it has just come out of the fridge, stir it up to help loosen the jam. Then, spoon about 1 teaspoon of jam into each indentation. Place the cookies in the fridge for at least 45 minutes.

Preheat oven to 375°F. Line two large cookie sheets with parchment paper. Place the chilled cookies on one of the cookie sheets about 2 inches apart. I was able to fit 12-13 cookies on each sheet. Bake the cookies, one sheet at a time, for 13-15 minutes, or until the cookies start to turn golden around the bottom edges.

Let the cookies rest on the cookie sheet for 15 minutes before transferring them to a cooling rack to cool completely before serving or storing in an airtight container.