Place butter in a bowl and beat until creamy.Scrape down the sides of the bowl, add sugars, and beat until ingredients are well-combined .Add egg yolk and vanilla extract and beat well.In a separate bowl, whisk together flour, cornstarch, and salt.gradually add flour mixture to wet ingredients until completely combined. This dough will seem very dry and crumbly, use your hands to finish working the dough together.Scoop cookie dough into 1 Tablespoon-sized balls and roll very well .
Roll in granulated sugar.Use your thumb or the rounded back of a teaspoon to gently press an indent in the center of the cookie dough. Repeat until all of the dough has been used.Transfer cookie dough to freezer and chill for 30 minutes.Once dough is done chilling, preheat oven to 375F .Spoon jam into each thumbprint, filling each indent to the brim.Once oven is preheated, place cookies at least 2" apart on a parchment paper-lined cookie sheet and bake on 375F for 15minutes or until edges are just beginning to turn golden brown.Allow cookies to cool completely on baking sheet before enjoying.
