Make the yeast-psyllium mixture. Sprinkle the yeast into a medium bowl. Add the milk and 5 grams of the sugar and whisk until dissolved. Let the mixture sit until frothy, about 10 minutes. Whisk in the psyllium vigorously. Let it gel for 5 minutes.

Make the dough. In a stand mixer, stir together the remaining 75 grams sugar, the potato starch, buckwheat flour, tapioca starch, salt, xanthan gum, and cardamom. Add the yeast-psyllium mixture. Snap on the dough hook and begin mixing the dough on medium speed. Add the eggs and egg white and continue mixing for 2 minutes until the dough comes together. Add the softened butter 1 tablespoon at a time and continue mixing for another minute. The dough will be soft, moist, and sticky, but should hold together.

Chill the dough. Lay a large sheet of plastic wrap on a work surface and scrape the dough onto it. Lightly dust the work surface with tapioca starch. Shape the dough into a rectangle that is roughly 6 by 7 inches (15 by 18 cm). The exact size of the rectangle is not important at this stage. Place the dough in the refrigerator and chill overnight.

Make the filling. In a medium bowl, stir together the brown sugar, soft butter, cardamom, and cinnamon until it is easy to spread and very creamy. You can also do this in a stand mixer with the paddle, but it should work well with a wooden spoon if your butter is soft. It is important that the butter be soft so it is easy to spread and doesn’t tear the dough. If your room is cold, microwave the butter for 5 seconds to warm it up. Set aside.

Roll the dough. Lightly dust a work surface and a rolling pin with tapioca starch. Remove your dough from the refrigerator, unwrap, and place it on the work surface. Knead the dough a few times to bring back some elasticity and then let it sit for 10 minutes to take the chill off. If the dough is too cold, it will crack while rolling. Gently roll the dough to a rectangle that is roughly 16 by 11 inches (41 cm by 28 cm). Move the dough around and dust with tapioca starch as needed so it doesn’t stick to your surface. It’s okay if the edges crack a little. Gently pull the corners out to create more of a rectangle than an oval, but don’t pull too hard or the dough may crack. There is little elasticity, so you must work gingerly.

Add the filling and roll. Line a sheet pan with parchment paper. Spread the filling all over the surface of the dough, trying not to tear it. With a short side facing you, fold the top third onto the middle and the lower third up over it, creating a letter fold. Rotate the dough 90 degrees. Dust the dough and rolling pin with tapioca starch, if needed, and roll the dough to a rectangle that is roughly 81/2 by 15 inches (22 by 38 cm). Transfer the dough to the lined sheet pan and refrigerate for 20 to 30 minutes.

Cut the dough. Remove the dough from the refrigerator and place it on your work surface with a short side facing you. Trim the edges to create sharp corners. Make 8 small marks 1 inch (2.5 cm) apart, at both ends. Use a long ruler to help you line up the top and bottom marks and cut the dough vertically into 8 strips that are roughly 1 inch (2.5 cm) wide and 15 inches (38 cm) long.

Shape the cardamom buns. Take one strip of dough and place it in front of you. Start twisting the dough in the middle toward one end and then twist toward the other end. Loosely coil the twisted dough into a spiral or a snail, tucking only the tip underneath at the end. Using both hands or a spatula, carefully transfer each spiral onto the lined sheet pan. Repeat with the remaining strips of dough until all the buns are shaped.

Proof the cardamom buns. Cover the pan with a linen towel or wrap it in a large plastic bag. Proof at room temperature until the buns feel lighter to the touch, about 40 to 45 minutes. 

Preheat the oven and make the cardamom sugar. While the buns are proofing, position a rack in the bottom third of the oven and preheat the oven to 450°F (230°C). In a small bowl, stir together the sugar and cardamom. Set aside along with the lightly beaten egg, which will be used to brush the tops.

Make the glaze. In a small saucepan, cook the sugar and water together over medium heat until the sugar is completely dissolved. Set aside.

Bake the cardamom buns. When the buns are proofed, lightly brush the tops with the beaten egg and sprinkle some cardamom sugar on top. Transfer the pan to the oven and reduce the oven temperature to 425°F (220°C). Bake for 15 minutes, or until the buns are a deep golden brown. Don’t worry if your buns crack in spots. There is not a lot of elasticity in the dough, so this can happen if you twist the buns too tightly. 

Glaze the buns. When the buns come out of the oven, brush them with the sugar syrup. Serve the buns while they are still warm. They can be reheated in the microwave for 20 seconds.