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1
Center oven rack to middle position and preheat oven to 350u00b0F. Line a baking sheet with parchment paper and set aside.
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2
In small mixing bowl, whisk together white rice flour, sweet rice flour, cocoa powder, tapioca starch, baking soda, salt, and xanthan gum.
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3
Fit stand mixer with paddle attachment. In bowl of stand mixer, cream together butter and brown sugar until butter-sugar paste lightens slightly, about one minute. (Use medium high-speed on a stand mixer. If using a handheld mixer, use high speed.) Turn off mixer. Scrape down sides and bottom of bowl. Add egg and vanilla extract. Turn mixer back on. Cream until egg is thoroughly incorporated, about one minute. Scrape down bottom and sides of bowl. Mix for 15 seconds.
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4
Add 1/3 of the dry ingredients. Mix until combined. Add half the buttermilk. Mix until batter is smooth. Add another 1/3 of the dry ingredients. Mix until combined. Add remaining buttermilk. Add remaining dry ingredients. Mix batter until smooth and thick, about 30 seconds.
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5
Scoop six mounds of batter, about 1/4 cup each, onto prepared baking sheet. Space cookies several inches apart as they spread during baking.
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6
Bake until cookies are set and spring back to the touch, about 18 minutes.
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7
Remove pan from oven. Allow cookies to cool on pan for one minute. Using a metal spatula, transfer cookies to wire rack to cool. Repeat with remaining batter.
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8
While cookies cool, make filling. (method below) Sandwich cooled cookies together with icing.
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9
In medium mixing bowl, cream together butter and confectioners' sugar until light and fluffy, about 30 seconds. (Use high speed on a handheld mixer or medium-high speed on a stand mixer.)
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10
Add marshmallow creme and vanilla. Mix until fluffy.
