Step 1: Prepare the Flour Blend
In a large mixing bowl, whisk together brown rice flour, white rice flour, potato starch, tapioca flour, and xanthan gum until completely combined with no lumps. Set aside.
Step 2: Activate the Yeast
In a small bowl, dissolve the yeast and 1 teaspoon of the granulated sugar in the warm milk (110°F). Let sit for 5-10 minutes until foamy and bubbly. If mixture doesn't foam, yeast is dead - start over with fresh yeast.
Step 3: Mix the Dough
Create a well in the center of the flour mixture. Pour in the foamy yeast mixture, remaining sugar, melted butter, beaten eggs, vanilla extract, and salt. Using a wooden spoon or sturdy spatula, mix from center outward, gradually incorporating flour until a soft, slightly sticky dough forms (about 3-4 minutes of mixing).
Step 4: First Rise
Transfer dough to a lightly oiled large bowl, turning once to coat all surfaces. Cover with damp cloth or plastic wrap. Place in warm, draft-free location for 1-1½ hours until doubled in size.
Step 5: Shape the Doughnuts
Gently punch down risen dough to release air. Turn onto lightly floured surface (use flour blend for dusting). Roll gently to ¾-inch thickness. Cut using floured doughnut cutter or two round cutters (3-inch outer, 1-inch inner). Place on parchment-lined baking sheets, spacing 2 inches apart.
Step 6: Second Rise
Cover shaped doughnuts with clean kitchen towel. Let rise 30-45 minutes until puffed and nearly doubled. Test readiness by gently poking - indentation should spring back slowly.
Step 7: Heat the Oil
Heat oil to exactly 365°F (185°C) in heavy-bottomed pot or deep fryer. Use thermometer to maintain temperature throughout frying.
Step 8: Fry the Doughnuts
Carefully slide 2-3 doughnuts into hot oil using slotted spoon. Don't overcrowd. Fry 1-2 minutes per side until deep golden brown. Remove with slotted spoon and drain on paper towels or wire rack.
Step 9: Make the Glaze
While doughnuts cool slightly, whisk together powdered sugar, milk, vanilla, and salt until smooth. Consistency should coat back of spoon but drip freely.
Step 10: Glaze and Serve
Dip warm (not hot) doughnuts face-down into glaze, allowing excess to drip off. Place on wire rack to set. Serve immediately or store in airtight container.