In the bowl of a stand mixer, whisk together the gluten-free flour, sugar, psyllium husk powder, instant yeast, and salt.
Warm the milk, butter, water and oil to 50°C. Slowly whisk in the egg until well combined. Recheck the temperature and warm again if needed to reach 50°C.
Add the warmed wet ingredients to the dry ingredients. Mix on medium speed for 2–3 minutes, scraping down the bowl as needed, until the dough is smooth and well combined.
Divide the dough into 4 equal portions (about 105 grams each). Gently shape each into a smooth ball.
Place the shaped buns about ½ inch apart on a parchment-lined baking sheet and press to flatten slightly into a disc shape.
Cover loosely with plastic wrap and let rise in a warm place for 45 minutes to 1 hour, or until visibly puffed.
Gently brush the tops with melted butter. Sprinkle with sesame seeds, if desired.
Gently brush the tops of the buns with melted butter. Sprinkle with sesame seeds, if desired.
Place an empty metal baking tray on the bottom rack of the oven while it preheats to 175°C.
Just before placing the buns in the oven, toss 5–6 ice cubes into the hot tray to create steam.
Bake the buns for about 30 minutes, or until golden brown and the internal temperature reaches around 95°C.
After the first 10 minutes of baking, you can remove the steam tray or simply let the steam dissipate naturally.