01 -
Beat softened butter using a stand or hand mixer in a large bowl until smooth and creamy.
02 -
Add granulated and brown sugars, then mix on medium-high speed for 30-60 seconds. Scrape the bowl sides occasionally.
03 -
Mix in the egg yolk and vanilla until nicely blended.
04 -
Blend together flour, salt, and cornstarch in a different bowl. Slowly add this to the wet ingredients. The dough will feel crumbly.
05 -
Roll dough into balls about 1 tablespoon each. Smooth them out by rolling between your hands. Add sugar coating if you'd like. Press the center with your thumb or a teaspoon.
06 -
Put the prepared dough balls into the freezer for a 30-minute chill.
07 -
Heat your jam just a little so it softens for easy spooning. Preheat oven to 375F (190C). Fill each cookie's center with jam.
08 -
Bake the cookies on a parchment-lined baking tray, leaving 2 inches in between, at 375F for about 11 minutes, or until edges start turning golden.