Mix the dry ingredients: In the bowl of a stand mixer, combine 160 g (1 cup) brown rice flour, 115 g (1 cup) tapioca starch, 75 g (½ cup) potato starch, 50 g (¼ cup) granulated sugar, 8 g (½ tablespoon) kosher salt, 7 g (2¼ teaspoons) instant yeast, 6 g (2 teaspoons) xanthan gum, and 8 g (2 teaspoons) baking powder. Whisk to combine evenly.
Make the psyllium gel: In a small bowl, whisk together 16 g (2 tablespoons) whole psyllium husk and 160 g (⅔ cup) warm water until thickened into a gel. Set aside for about one minute to hydrate fully.
Combine the wet ingredients: In a medium bowl, whisk together 160 g (⅔ cup) warm whole milk, 1 large egg, 57 g (4 tablespoons) avocado oil, and 5 g (1 teaspoon) apple cider vinegar. Add the psyllium gel and whisk until smooth and uniform.
Make the dough: Pour the wet ingredients into the dry ingredients. Using the dough hook attachment, mix on medium speed for about 5 minutes, scraping the sides and bottom of the bowl as needed. The dough should be soft, thick, and slightly sticky.
Shape the buns: Lightly dust your work surface and hands with tapioca starch. Turn the dough out onto the surface and knead it a few times until smooth.
Divide into 6 equal pieces, about 140g each.
Fold the edges of each piece toward the center, pinch the seams closed, and flip seam side down.
Roll each piece under your hand in a loose claw shape until round and smooth. Gently press each ball down to about 1 inch in height.
Arrange the buns on a parchment lined baking sheet, spacing about 2 inches apart. Cover loosely and proof in a warm spot at 75 to 80°F for 1.5 to 2 hours, until puffy and nearly doubled. Preheat the oven to 400°F during the last 30 minutes of the proof time.
Brush the tops of the buns with 1 large egg and sprinkle with 1 tablespoon sesame seeds.
Bake at 400℉ for 16-17 minutes until deep golden brown. Check the internal temperature with a thermometer. The buns are done when it reaches 190°F to 195°F.
Let cool on the baking sheet for a few minutes before transferring to a wire rack.