For the Gluten-Free Flour Blend: Whisk the rice flour, tapioca starch, sorghum flour, cornstarch, oats and guar gum together in a large bowl. Be sure to use the scoop-and-sweep method when measuring out your gluten-free flours and starches, as the starches can really get packed into measuring cups, resulting in a very high ratio of error. I like to fluff all of my ingredients with a fork before I begin. Measure out what you will need for the cookies, and store any extra flour in an airtight container for up to 2 years.
For the Cookies: Sift 1 1/2 cups Gluten-Free Flour Blend with the cocoa and baking soda. Gently tap any oats that might remain in the strainer over the sifted ingredients.
In a stand mixer with the paddle attachment, cream the butter until very smooth, then add the light brown sugar, granulated sugar and kosher salt and mix until pale and fluffy. Scrape down the sides of the bowl, add the egg and mix on high until incorporated. Add the sifted ingredients and mix until the ingredients are sticking to the sides of the bowl and paddle.
Scrape down the sides of the bowl again, then add the vanilla and vodka, mixing until the dough is nice and thick. Remove the bowl from the mixer and, using a rubber spatula, stir in the semisweet chocolate.
Transfer the dough to a clean bowl, cover with plastic and refrigerate for at least 24 hours and up to 36 hours for the best results. This allows the sugars in the dough to absorb the liquids from the eggs, vanilla, butter and vodka, resulting in a cookie that has a heavenly crunchy perimeter and a delicate cake-like center.
When the dough has finished chilling, line 2 cookie sheets with parchment paper and preheat the oven to 350 degrees F.
Using a 1 1/2-inch retractable scooper, measure out a round of dough and level it in the scooper. Roll the dough between your hands, then gently flatten the ball into a 2-inch disk. Place on the prepared cookie sheet. Repeat until all of your cookies have been made, being sure to leave at least 2 inches between the cookies, as they will grow and spread slightly while baking.
Bake until the cookies are set and no longer glossy, 7 to 8 minutes, rotating the cookie sheets halfway through for the most even bake.
Allow the cookies to cool on the cookie sheets for at least 5 minutes, as they are delicate right out of the oven. Using a spatula, transfer the cookies to a wire rack to cool completely. While the cookies cool, make the ganache filling, if using.
For the Ganache Filling: Put the chopped chocolate in a heatproof bowl. Heat the heavy cream in a saucepan until it steams, then pour it over the chocolate. Allow the cream to sit for 3 to 5 minutes to melt the chocolate. Whisk until smooth, then stir in the salted butter and light corn syrup. Add 1 to 2 tablespoons of vodka into the ganache and whisk just to combine. Set aside so the ganache can set for at least 30 minutes.
Spread a small amount of the ganache on the back of one of the cookies and then press a second cookie over the chocolate to make a sandwich.
Recipe courtesy of Karen Morgan, Blackbird Bakery