First, Bake The Cookies:

1. Preheat The Oven & Prep Your Pans. Start by preheating the oven to 350 degrees F. Line two baking sheets with parchment paper. (This way you can alternate baking sheets between batches & not put cool dough on a hot baking sheet.)
2. Cream Butter & Sugars. In the bowl of a stand mixer with a paddle attachment (or a large bowl with a hand mixer), cream the softened butter, sugar, and brown sugar 2-3 minutes, or until smooth, light, and fluffy.
3. Add Egg Yolk & Vanilla. Mix in the egg yolk and vanilla extract, until well blended.
4. Add Dry Ingredients. Scrape down the edges of the bowl and add gluten-free flour, baking soda, and salt. Mix until just a few floury patches remain.
5. Mix In Chocolate Chips. Add mini chocolate chips and mix until just combined and no floury patches remain.
6. Scoop & Roll. Use a small cookie scoop to form 1 Tablespoon balls. Roll the cookie dough balls smooth between your palms quickly, then add 6-8 cookie dough balls to a baking sheet.
7. Bake & Cool. Bake the cookies 8-10 minutes or until set on the edges and just underdone in the center. If desired, shape the cookies into perfect circles with a round cookie cutter and press a few extra chocolate chips into the tops right after you pull them out of the oven (this is just for looks!). Let the cookies cool 2-3 minutes on the baking sheet before transferring to a wire rack to cool completely. Repeat baking process until all the cookie dough is baked. (Note: these cookies are meant to be thin, but if they’re spreading WAY too much, you can chill the dough for 20 minutes to help control the spread)
8. Freeze. Place the cookies on a baking sheet lined with parchment (feel free to reuse one from baking!) and freeze at least 30-60 minutes to allow the cookies to harden. (This helps them not break when you spread the ice cream). (You can also transfer cookies to an airtight bag and store in the freezer up to 2-3 weeks in advance at this point.)

Then, Build The Ice Cream Sandwiches. 

1. Gather All Your Ingredients. Before you start, get everything ready to go, so you can move quickly. The ice cream will begin to melt as soon as you scoop it, so you don’t want to waste any time! Set out a shallow plate or bowl with any toppings you’d like to use (chocolate chips, nuts, sprinkles, etc.). Remove the frozen cookies from the freezer and set out a tray or plate for the finished ice cream sandwiches that you can store in the freezer. 
2. Work ONE Sandwich At A Time. Since the ice cream will melt, only work with one sandwich at a time.
3. Scoop & Roll. Use a standard 1/4-cup ice cream scoop to make 1/4 cup scoops of ice cream. Add the scoop of ice cream to the bottom of one cookie, then cover with the bottom of the second cookie (so the tops of the cookies face outward). Gently spread or press the ice cream to flatten slightly. If your ice cream is very stiff, it can be helpful to use an offset spatula to gently press or spread the ice cream out on the bottom cookie before adding the top cookie.
4. Decorate. Sprinkle or roll the sides of your ice cream sandwich with toppings and transfer to the plate or tray in the freezer. Repeat with your remaining cookies and ice cream until you’ve finished making all the ice cream sandwiches.
5. Freeze Again! For the best texture, freeze the ice cream sandwiches at least 2-3 hours before serving.
6. Serve & Store. When the sandwiches are frozen, you can wrap the individually in plastic wrap or parchment and store in an airtight container or freezer bag in the freezer up to 2 months.