65 g Expandex® modified tapioca starch (~1/2 cup)*
6 ounces finely chopped dark chocolate, melted (we like Scharffen Berger bittersweet chocolate baking chunks)
Preheat oven to 325°F.
Line 2 large baking sheets with parchment paper and put a third sheet of parchment paper on the counter. Set aside.
Pulse flours, sugar and salt in a food processor until combined. Add cold butter and pulse in 1-second intervals 8-10 times or until mixture looks like coarse breadcrumbs. With machine running, add vanilla then 1/4 cup + 1 tablespoon heavy cream through feed tube until mixture just comes together into a dough, adding more cream by the teaspoonful if mixture looks dry. Dough should form several clumps and should look smooth.
Sprinkle third sheet of parchment with sweet rice flour and turn dough out from the bowl of the food processor. Sprinkle dough with a little more sweet rice flour and gather into a ball. Flatten slightly and roll out to an even 1/8-inch thickness with a floured rolling pin. If dough sticks, sprinkle with the smallest amount of sweet rice flour possible to prevent sticking.
Using a floured cookie cutter (approx. 2 1/2-inches diameter; we used a cute flower shape), cut rounds out of rolled out dough and place onto parchment-lined cookie sheets about 1 inch apart. Regather dough and repeat the rolling and cutting process until all the dough is used.
Place baking sheets in freezer for 15 minutes, then remove from freezer, and brush lightly with egg yolk mixture.
Place baking sheets in middle of oven and bake 20-25 minutes, rotating baking sheets halfway through baking time, until deep golden brown in color. Cool on baking sheet 5 minutes, then transfer to a wire rack to cool completely.
Spread melted chocolate over the bottom side of half of the cookies. Top with remaining cookies and refrigerate until set, about 15 minutes.
Cookies will keep in an airtight container for up to 1 week, but they will soften over time.
*Expandex® modified tapioca starch can be found in some retail stores and can be purchased online (look here for a where-to-buy guide); it enhances the texture and appearance of gluten-free baked goods. And if you're curious about why the ingredients are measured by weight, read our post about the success of baking gluten-free using weights vs. measures here).