Oil a 9-inch round cake pan; set aside.

In the bowl of your stand mixer, combine potato starch, cornstarch, rice flour, sorghum flour, non-fat milk powder, flax seed meal, yeast, baking powder, xanthan gum, baking soda, and salt.

In a separate bowl, whisk together the yogurt, honey, oil, eggs, water, and vinegar.

Add the wet mixture to the dry mixture and beat on high speed using the whisk/balloon attachment until thoroughly combined, 1 minute.

Oil a large bowl. Transfer the dough to the oiled bowl and either roll it around in the oil or spray the top and sides of the dough with oil so that it doesn't become dry. Cover the bowl with a tea towel and allow to rise in a warm environment, 80 - 100°F, until doubled in size, about 45 minutes.

Once the dough rises, lay out two overlapping sheets of plastic wrap about 15 x 20-inches and dust generously with gluten-free all-purpose flour or potato starch. Scoop the dough onto the dusted plastic wrap, dust with flour, and pat out a little. Then roll the dough out with a flour dusted rolling pin to about 11 x 15-inches. Allow the dough to rest while you prepare the filling.

In a small bowl, mix together the brown sugar and cinnamon; set aside.

Baste the 11 x 15-inch dough with melted butter, sprinkle with the brown sugar-cinnamon mixture making sure to place some filling to the very ends of the long edges. Leave a 1/2-inch border on the shorter edges.

Lift up the plastic wrap and roll the dough starting with the shortest end to make an 11-inch long log. Transfer the log to the prepared pan and pinching the ends to form a circle.

Set the pan aside to rise in an approximately 80°F environment for 30 minutes or until the cake fills at least three-quarters of the pan. While rising, preheat the oven to 400°F with a rack in the center of the oven.

Once the cake rises, bake 15 minutes.

While the cake bakes, if you desire the cake to brown and have a bit of crisp outer texture, combine the egg, milk, and honey; set aside.

Also while the cake bakes, whisk together the confectioners' sugar, heavy whipping cream, and vanilla for the glaze; set aside.

After 15 minutes of baking, if using the egg wash, using a pastry brush, baste the top of the cake with the egg wash mixture and bake an additional 5 minutes or until a toothpick inserted in the center of the cake comes out clean.

Remove the cake from the oven. Melt 1 tablespoon of butter and baste the tops of the cake. This softens the crispness of the honey egg wash and provides additional flavor and moisture.

Freeze leftovers in a single layer once the glaze sets and the cake is completely cooled. Once frozen, store in a resealable storage bag. Microwave on low to reheat.