1-Cream the softened butter using a stand mixer or hand mixer until smooth.

2-Gradually add and beat in the sugars, increasing speed to medium-high and mixing for 30-60 seconds until fully combined.

3-Mix in the egg yolk and vanilla extract thoroughly.

4-In a separate bowl, whisk together the flour, cornstarch, and salt.

5-Slowly incorporate the dry ingredients into the wet mixture on low speed, scraping the bowl’s sides as needed. Finish mixing by hand if necessary.

6-Scoop dough into 1 tablespoon-sized balls and roll to form smooth spheres without cracks.

7-Optionally, roll dough balls in sugar, then place them on wax paper or a small cookie sheet.

8-Press a gentle indent into the center of each dough ball using your thumb or a rounded teaspoon.

9-Chill the dough in the freezer for 30 minutes.

10-Preheat the oven to 375°F (190°C).

11-Warm the jam slightly in the microwave for 5-10 seconds until fluid but not hot.

12-Fill each indentation with the warmed jam.

13-Place the cookies on a parchment-lined baking sheet with at least 2 inches between them.

14-Bake for 11 minutes or until the edges just begin to turn golden.

15-Allow cookies to cool completely on the baking sheet before serving.