• Grease a 9-inch x 13-inch casserole dish and set it aside.

Make the dough

• In the bowl of a stand mixer fitted with the paddle, whisk together the flour, xanthan gum, tapioca starch, milk powder, yeast, cream of tartar, baking soda, and sugar. Add the salt and whisk again.
• Add the vinegar, butter, egg, and milk, and mix to combine.
• Increase the mixer speed to high and beat for about 3 minutes, until the dough is smooth and pulling away from the sides.
• Transfer the dough to a sealed container and refrigerate for 30 minutes to make it easier to handle.

Roll out the dough

• Turn the chilled dough onto a lightly floured surface and knead gently to smooth.
• Divide in half, keeping one half covered. Roll the other into a 9x12-inch rectangle about ½ inch thick, dusting with flour as needed.

Add the filling and shape

• Spread half the softened butter over the dough, leaving a 1-inch border. Sprinkle with half the brown sugar, pat it down, then sprinkle with cinnamon.
• Starting from a short side, roll the dough tightly into a coil. Slice into 6 rolls using floss or a sharp knife.
• Place in a greased 9x13-inch pan, 1 inch apart. Press each roll down about ⅓ of the way to flatten slightly.
• Repeat with the second half of the dough and filling.

Let rise

• Cover the pan with lightly oiled plastic wrap. Let rise in a warm spot until the rolls are about 50% larger—about 45 minutes.

Bake

• Preheat oven to 350°F.
• Remove plastic wrap and bake for 20 minutes, until risen, lightly golden, and baked through.
• Let cool for 5 to 10 minutes before icing or frosting, but no longer.

Optional glaze.

• Whisk together confectioners’ sugar and milk until thick but pourable. Add more milk ¼ teaspoon at a time if needed.
• Drizzle over warm rolls.

Optional frosting

• Beat butter and confectioners’ sugar until smooth. Add cream gradually to reach a spreadable consistency.
• Spread generously over warm rolls.