01 -
Using a stand mixer or electric hand mixer, whip the butter in a big bowl until it's light and airy.
02 -
Mix in both sugars, scraping the bowl sides, and beat on medium speed for 30-60 seconds until smooth.
03 -
Toss in the egg yolk and vanilla extract and blend it all thoroughly.
04 -
In another bowl, stir together the flour, salt, and cornstarch. Slowly add this to the butter mixture until it forms a crumbly dough.
05 -
Form the dough into 1-Tablespoon balls, rolling until smooth. If you'd like, roll them in sugar. Use your thumb or a small spoon to press an indent in the center.
06 -
Pop the dough balls into the freezer for half an hour to firm up.
07 -
Preheat oven to 375F (190C), then gently heat up the jam so it spreads easily. Spoon it into the indents.
08 -
Line a baking sheet with parchment paper, place cookies 2 inches apart, and bake at 375F for around 11 minutes. The edges should just be turning golden.