Mix the dry ingredients: In the bowl of a stand mixer.
Make the psyllium gel: In a small bowl, whisk together whole psyllium husk and 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 warm whole milk, 1 large egg, avocado oil, and 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 8 equal pieces, about 105g each. Pat each piece into a roughly 3×5 inch rectangle.
Starting from the short side, roll the dough up tightly into a cylinder. Pinch the seams closed, then roll gently until the bun is about 5 inches long.
Arrange the buns on a parchment lined baking sheet in two groups of four, spacing them less than half an inch apart within each group so they are almost touching.
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 poppy 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.