For the sponge: In the bowl of your stand mixer, combine the sponge ingredients and let it rest for 15 minutes.
Add the pineapple juice, butter, brown sugar, eggs and yolk, and vanilla, mixing until well combined. In a separate bowl, whisk together the remaining flour, potato flour, and salt, then add this mixture to the liquid ingredients.
Mix and knead until the dough is cohesive and smooth; it will be very sticky at first. Beat with the flat beater of your stand mixer at medium-high speed for about 3 minutes. Scrape the dough into the center of the bowl, switch to the dough hook, and knead at medium speed for about 5 minutes. The dough may form a very soft ball but will likely still stick to the bottom of the bowl.
Lightly grease the mixing bowl, round the dough into a ball, and place it in the bowl. Cover and let it rise until very puffy, about 1 ½ to 2 hours.
Lightly grease a 9" x 13" (23 cm x 33 cm) baking pan. Gently deflate the dough and divide it into 15 equal pieces. Round each piece into a smooth ball and space the buns in the pan. Cover the dough gently with lightly greased plastic wrap and let it rise in the pan for 1 hour, until nicely puffy. Toward the end of the rising time, preheat the oven to 350°F (176°C).
Mix the reserved egg white with 1 tablespoon of cold water and brush some onto the surface of the rolls; this will give them a satiny crust. Bake the rolls for 20 to 25 minutes, or until they are cooked through. Remove the rolls from the oven and let them cool for 5 minutes before turning them out onto a cooling rack. Serve warm with butter or your favorite preserves.