1-Using the “scoop and swipe” method to: Using the “scoop and swipe” method to measure the flour, sift flour, cornstarch and salt together in a small bowl then set aside.
2-Beat butter with an electric mixer on: Beat butter with an electric mixer on medium speed until light and creamy. Add white sugar and mix until well combined. Add brown sugar then continue mixing on medium speed until fluffy (about 3 minutes), scraping down the sides occasionally.
3-Mix in egg yolk and vanilla extract: Mix in egg yolk and vanilla extract.
4-Mixing on low speed, add flour mixture,: Mixing on low speed, add flour mixture, 1/2 cup at a time, to the butter mixture. The batter will be very stiff but will eventually come together.
5-Using a small cookie scoop or tablespoon: Using a small cookie scoop or tablespoon to measure, roll dough tightly into small balls (about a tablespoon-sized portion). Roll dough balls in white sugar then place on a lined cookie sheet (parchment or silpat mat). *See notes below about placement.
6-Using a small round measuring spoon (or: Using a small round measuring spoon (or your thumb) slowly press an indentation into each dough ball. Freeze dough for 45 minutes.
7-Preheat oven to 375 degrees: Preheat oven to 375 degrees.
8-Spoon jam or preserves into small bowls: Spoon jam or preserves into small bowls then whisk until smooth. If necessary, heat each in the microwave for about 20 seconds or until they can be whisked smooth.
9-Fill each cookie indention with desired jam: Fill each cookie indention with desired jam or preserves. You can fill them just to the top but not more.
10-Bake cookies at 375 degrees for 10-12: Bake cookies at 375 degrees for 10-12 minutes or until just starting to brown around the bottoms.
11-Cool for at least 15 minutes on: Cool for at least 15 minutes on the pan (or completely) then store in an air-tight container up to one week.