Combine Dry Ingredients: In the bowl of a stand mixer, add the brown rice flour, potato starch, tapioca flour, xanthan gum, light brown sugar, fine sea salt, instant yeast, and whole psyllium husks. Whisk briefly to combine.

Add Wet Ingredients: Add the mashed potatoes, whole eggs, egg yolks, softened cultured butter, and buttermilk to the dry ingredients.

Mix and Knead: Using the dough hook attachment, mix on low speed until a shaggy dough forms. Increase the speed to medium and knead for 5 minutes. The final dough will be very soft and slightly sticky.

First Rise: Scrape down the sides of the bowl, cover it with plastic wrap or a clean towel, and let it rise in a warm, draft-free place for 1–2 hours, until noticeably puffed up.

Chill the Dough: After the first rise, place the covered bowl in the refrigerator to chill for at least 6 hours, or ideally overnight. This step is crucial for flavor and texture.

Prepare for Baking: When ready to bake, remove the dough from the fridge and let it rest at room temperature for 30 minutes. Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.

Shape the Buns: Lightly dust a work surface with rice flour. Turn the chilled dough out and knead it gently a few times. Divide the dough into 8 equal portions (about 90-95 g each). To shape, tuck the edges of each portion underneath itself to form a taut surface, then roll it into a smooth ball.

Second Rise: Place the shaped buns on the prepared baking sheet, leaving about 1-2 inches between them. Cover loosely with plastic wrap and let them rise in a warm spot for 25–30 minutes, until they look puffy.

Bake: Bake for 15–20 minutes, or until the tops are a deep golden brown and the buns sound hollow when tapped on the bottom.

Add the Finishing Touch: For a soft, rich crust, brush the tops of the hot buns with melted butter as soon as they come out of the oven (optional).

Cool: Transfer the buns to a wire rack to cool completely before slicing and serving.