In a medium bowl, whisk together the almond flour, oat flour, sweet rice flour, corn starch, baking cocoa, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment or a mixing bowl with a hand mixer, beat the butter, baker’s sugar, and light brown sugar until light and fluffy, 3-5 minutes. Add the chocolate extract and mix until combined.
With the mixer on low, slowly add the dry ingredients to the butter beating until well incorporated but still crumbly.
Divide the dough in half and form into two discs. Roll out each disk between two pieces of parchment paper to 1/8-1/4 inch thickness or a 12x11-inch oval. Chill on a baking sheet for 15 minutes.
Working with one half at a time, use a 2-inch round or fluted cutter to cut each round into cookies. Roll the scraps up and cut out cookies again – try to get the cookies as close together as possible, so you only had to roll out the scraps 2 times or else the cookies will get more dense. And tough.
Chill the cookies on a parchment-lined baking sheet in the fridge for 30 minutes or in the freezer for 15 minutes.
Meanwhile preheat the oven to 350°F. One sheet at a time, bake the cookies on a parchment-lined baking sheet 1.5-inches apart for 10-11 minutes.
When the cookies come out of the oven, use the bottom of a glass cup to gently press them down so they’re flat and smooth on top. Let cool on the baking sheet for 5 minutes before using a thin metal spatula to transfer them to a wire rack to cool completely before filling. Repeat with remaining baking sheets.
While the cookies cool, make the buttercream. In the bowl of a stand mixer fitted with the paddle attachment or in a mixing bowl with a hand mixer, cream the butter over medium speed until light and fluffy, 3-5 minutes.
With the mixer on low, slowly add the powdered sugar, vanilla extract, and, if using, ¼ teaspoon peppermint extract, and salt. Beat until combined. Taste and add more peppermint extract as desired. Fill a piping bag fitted with a ½-inch round pastry tip with the frosting.
Pair the cookies by similar size. Pipe a dollop of frosting onto half the cookies, then gently make a sandwich with the paired cookies. Chill cookies for 20 minutes to set the frosting before eating (or the frosting will ooze out when you bite down!).
