Mix the dry ingredients: In the bowl of a stand mixer, combine 160g brown rice flour, 115g tapioca starch, 75g potato starch, 50g sugar, 8g salt, 7g instant yeast, 6g xanthan gum, and 8g baking powder. Whisk to combine evenly.
Make the psyllium gel: In a small bowl, whisk together 16g whole psyllium husk and 160g 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 160g warm milk, 1 egg, 42g melted butter, and 5g apple cider vinegar. Add in 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.
First roll: Lightly dust your work surface with tapioca starch. Turn out the dough and pat into a rough rectangle. Roll into a 10×14-inch rectangle about ¼-inch thick.
Add the frozen butter: Using a box grater, grate 42g (3 tablespoons) frozen butter evenly over the entire surface of the dough, leaving no bare spots.
First letter fold: Fold the right third of the dough over the center, then fold the left third over that. Give the dough a quarter turn so the whole thing is now horizontal in front of you.
Second roll and fold: Roll the dough back out into a 10×14-inch rectangle. Fold the right third over the center, then fold the left third over that. The dough should feel cool but pliable.
Roll into cylinder: With your hands, roll up the folded dough from the short side into a cylinder (it will look like a massive cinnamon roll) to distribute the butter layers evenly, then roll out with the rolling pin into a 14-inch circle about ⅛-inch thick.
Cut the crescents: Using a pizza cutter or sharp knife, cut the circle into 12 equal wedges (like cutting a pizza).
Shape the rolls: Starting from the wide end of each wedge, roll toward the point. Place seam-side down on a parchment-lined baking sheet, leaving about 2 inches between each roll. Curve the ends slightly inward to create the classic crescent shape.
Proof the rolls: Cover loosely and let rise in a warm spot (75–80°F) for about 45–60 minutes, or until puffy and nearly doubled in size.
Bake: Preheat the oven to 375°F (190°C). Brush the tops with 1 beaten egg for shine. Bake for 18-22 minutes, or until golden brown and the centers reach 200°F.
Finish: While still warm, brush the tops with 14g (1 tablespoon) melted butter and sprinkle lightly with flaky sea salt. Let cool for a few minutes before serving.