In a medium bowl, sift together the white rice flour, oat flour, sweet rice flour, tapioca starch, xanthan gum, baking powder, and salt. Set aside.
In the bowl of a stand mixer, cream the butter until smooth. Slowly add the powdered sugar and beat until light and fluffy.
Add the vanilla extract and mix until combined.
Gradually add the dry ingredients to the creamed butter, beating until it comes together in a ball.
Divide the dough into two balls. Wrap them in plastic and refrigerate for 1 hour.
Working with one ball at a time, roll out the dough onto a lightly floured surface until it is about ¼-inch thick.
Use a pastry wheel or a knife to cut 2-½ inch squares. Carefully transfer each square to a parchment-lined baking sheet, 1 inch apart. Rework any leftover scraps back into the dough as you would with sugar cookies. Repeat until you have 24-30 cookies.
Use a skewer to poke 5 evenly spaced holes in the top of each cookie.
Bake the cookie sheets side-by-side for 16-18 minutes, or until they spring back when lightly touched in the center (they'll crisp up as they cool).
Turn oven off. Open the oven door and let the cookies rest until the oven is cool, or at least 20 minutes. Store in an airtight container for up to one week.
S'mores
Spread about 2 teaspoons of lemon curd on one cooled shortbread cookie.
Using either a fire or the flame from a gas stove, roast a marshmallow. (See notes below).
Press the roasted marshmallow between the lemon curd covered shortbread cookie and another shortbread cookie. Devour instantly.
Notes
Homemade marshmallows roast a bit differently than store-bought marshmallows. I tried roasting homemade marshmallows under the broiler and in the microwave for just 5 seconds and both attempts led to completely MELTED marshmallows. I tried roasting with a blow torch, but found the outside caramelized while the inside didn't cook. I had the best luck roasting homemade marshmallows over the flame of a gas stove or over an open fire.If you're using store-bought marshmallows, you can roast them indoors easily under the broiler, or for the super quick way, microwave them for 5-10 seconds until they puff up.
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.