In the bowl of a stand mixer (or use a large bowl with an electric hand mixer), add the butter. Beat for 2 minutes until light and fluffy.
Add the granulated sugar and light brown sugar. Mix on medium-high speed until light and fluffy, another 1-2 minutes.
Add the egg yolk and vanilla and mix to combine.
Add the gluten-free flour, cornstarch and salt. Mix on low speed, then after 30 seconds increase the speed to medium. Mix for 2-3 minutes until a thick dough forms.
Line a flat plate or small baking sheet with parchment paper. (One that will fit in your freezer!) Using a one-tablespoon cookie scoop or measuring tablespoon, measure out the cookie dough, roll into balls, roll each one in granulated sugar (optional), then place on the parchment-lined plate.
Use your thumb (or the handle of a large wooden spoon) to press an indentation into each ball of cookie dough. Smooth any seams or cracks on the edges of the cookie.
Freeze the cookie dough balls for 20 minutes.
Spoon a scant 1/2 teaspoon of preserves/jam into the indentation in each one. (You can make this easier by scooping the preserves into a small ziplock bag and clipping the edge of the bag, then piping a little bit into each cookie. Do not overfill.)
Freeze for another 15-20 minutes. Preheat the oven to 375F. Line a baking sheet with parchment paper or a silicone baking mat.
Place the cookies at least 2" apart on a parchment paper-lined cookie sheet and bake for 12 minutes, until starting to turn golden brown.
Let the cookies cool on the baking sheet for 10 minutes. Then transfer to a wire cooling rack. Repeat baking the remaining dough. Enjoy!