Preheat oven to 350 degrees. Butter a 13 by 9-inch baking dish. In the bowl of an electric stand mixer, whisk together yeast, 1/2 tsp of the granulated sugar (from the 1/2 cup) and the water. Let rest 5 minutes.
Meanwhile, in a mixing bowl, whisk together rice flour, potato starch, tapioca flour, xanthan gum, baking powder, baking soda and salt for 30 seconds, set aside.
To the proofed yeast mixture, add warm milk, the remainder of the 1/2 cup of sugar, 1/4 cup butter, the canola oil and eggs and blend mixture with paddle attachment to combine (mixture will be slightly lumpy but will smooth when dry ing. are added).
With mixer set on low speed, slowly add in dry ingredients. Once dry ingredients have been incorporated, increase mixer speed to medium-low and mix 2 minutes.
Spread two long sheets of plastic wrap onto countertop one slightly overlapping the other.* Dust plastic wrap lightly with rice flour then drop dough mixture onto plastic wrap. Sprinkle top of dough lightly with rice flour then spread two more long sheets of plastic wrap over dough, then roll dough out into a 16 by 13-inch rectangle.
For the filling: spread 6 Tbsp partially melted butter over dough, then in a bowl, whisk together brown sugar, granulated sugar, cinnamon and 1/8 tsp salt and sprinkle evenly over melted butter (leaving a small rim uncovered along all edges).
Starting on the 16-inch side, roll dough snuggly to opposite side using plastic wrap to lift and roll the dough, and tucking as needed. Press end seam together with fingertips. Cut dough into 12 equal portions (don't saw back and forth with knife, just do one cut downward).
Carefully lift rolls to prepared baking dish, aligning them side by side.** Bake in preheated oven 22 - 25 minutes. Cool slightly then frost with cream cheese icing.
For the icing: In the bowl of an electric stand mixer (or using a hand mixer), whip together cream cheese and butter until smooth and fluffy. Add powdered sugar, sour cream and vanilla and whip until fluffy, about 2 minutes longer.
Best served day prepared. Store in an airtight container and re-warm individually in microwave 10 - 20 seconds (as they are best served warm).
Notes
*You will be using it to roll the dough up since it is sticky and rather fragile. You'll need the plastic wrap it to be wide enough so you'll likely have to use two sheets unless you have really wide plastic wrap because it needs to be at least 15 inches wide - for the 13-inch side since you need to leave at least a 1-inch rim on edges for rolling.
**You'll have a little extra space on one end which is fine, you can either tuck a rolled up piece of greased foil there or just leave it and the rolls on that end will spread a little more.
If on a gluten-free diet, be sure to check all of the packaging labels on ingredients to make sure they are gluten-free.