For the gluten-free flour blend: In a large bowl, whisk together the rice flour, tapioca flour, potato starch, xanthan gum and salt.
For the buns: In the bowl of a stand mixer fitted with the paddle attachment, mix together the potato flakes, dry milk, sugar, yeast, salt, 2 tablespoons butter and 1 3/4 cups plus 2 tablespoons gluten-free flour blend on medium-low speed. With the machine running, stream in the buttermilk and mix until the dough is combined and pulls away from the sides of the bowl.
Line your countertop with a large piece of parchment paper and sprinkle lightly with some of the gluten-free flour blend. Grease an 8- by 8-inch baking dish. Roll the dough out gently to about 3 inches thick. Cut the dough into 8 equal pieces and place in the prepared baking dish, evenly spaced from one another. Cover with plastic wrap and let rise until doubled in size, about 1 1/2 hours.
Preheat the oven to 350 degrees F. Bake the buns until golden and fluffy, about 24 minutes; remove from oven and brush with butter. Serve warm or cooled.