Wet Ingredients: To a large mixing bowl, add the butter, sugar, brown sugar, milk, and vanilla extract. Mix with an electric mixer until well combined.
Dough: To a medium mixing bowl, add the gluten-free flour, potato starch, and salt. Whisk together. Then add half the dry ingredients to the butter mixture and mix until smooth, scraping down the bowl as needed. Add the remaining dry ingredients and mix the dough thoroughly.
Balls: Scoop 1 tablespoon of dough and roll well into a ball without cracks or blemishes (this prevents the cookies from cracking while baking or pressing the thumbprint). If the dough is too soft, put it in the refrigerator very briefly to firm up so it's manageable. It's best to keep the dough soft so it doesn't crack when pressing thumbprints. Roll the ball in sugar. Repeat with the remaining dough. Place the dough balls on a cookie sheet lined with parchment paper with about 2 inches between cookies.*
Thumbprints: Using a 1 teaspoon measure or your thumb, gently and slowly press down into the dough balls to create the thumbprint. This will cause the dough to spread. The thumbprint depth should be about ⅓ (and no more than half) of the cookie height. Repair any cracks by gently smoothing the dough with your fingers. Place the indented dough balls in the freezer for 30 minutes to firm up.
Jam: Just before taking the cookies out of the freezer, preheat the oven to 375 degrees F. Warm the jam in the microwave for 10-15 seconds and stir until smooth. Take the cookie dough out of the freezer and fill each thumbprint to the brim with jam.
Bake: Bake for 12 minutes or until the edges are just turning golden brown. Allow the thumbprint cookies to cool completely on the pan.
Notes
See the blog post for step-by-step photos, gluten-free ingredient brands, expert tips and more.Storage: Store leftover gluten-free thumbprint cookies in an airtight container at room temperature for up to 4 days. You can also store them in the refrigerator for up to a week or in the freezer for up to 3 months.Small Freezer: If your cookie sheet won't fit in the freezer, you can press the thumbprints on wax paper and transfer the cookies to the cookie sheet after freezing.Soft Dough: If the dough is too soft to work with, refrigerate for 5-10 minutes at a time and try again. The softer it is, the easier it is to prevent cracking.