Sponge: In the bowl of your stand mixer or the bucket of your bread machine, combine the sponge ingredients. Allow the sponge to rest for 15 minutes. Add the pineapple juice, butter, brown sugar, eggs and yolk, and vanilla, mixing until well combined. Whisk together the remaining flour, potato flour, and salt before adding to the liquid ingredients. Mix and knead until the dough is cohesive and smooth; it'll be very sticky at first. If you're using a stand mixer, beat with the flat beater for about 3 minutes at medium-high speed; then scrape the dough into the center of the bowl, switch to the dough hook, and knead for about 5 minutes at medium speed. It may have formed a very soft ball, but will probably still be sticking to the bottom of the bowl. If you're using a bread machine, simply let it go through its entire cycle. If you find the dough isn't coming together, add a tablespoon or two of flour. Lightly grease the mixing bowl or a large (8-cup) measure; round the dough into a ball, and place it in the bowl or measure. Cover, and let rise until very puffy, about 1 1/2 to 2 hours. If you're using a bread machine and the dough hasn't doubled in size when the cycle is complete, simply let it rest in the machine for another 30 to 60 minutes. Lightly grease a 9" x 13" pan. Gently deflate the dough. Divide it into 16 equal pieces, by dividing in half, then in halves again, etc. Round each piece into a smooth ball. Space the buns in the pan. Tent the dough gently with lightly greased plastic wrap. Let the dough rise in the pan for 1 hour, until it's nicely puffy. Toward the end of the rising time, preheat the oven to 350°F. Mix the reserved egg white with 1 tablespoon 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 the internal temperature reads 190°F on a digital thermometer. Remove the rolls from the oven, and after a few minutes, turn them out onto a cooling rack. Serve warm. Store leftovers, well wrapped, at room temperature for several days; freeze for longer storage. Yield: 16 buns. Chat with a baker. Tips from the bakers. Sweet breads can be agonizingly slow risers. Why? Because sugar attracts water, and when it's in bread dough, it pulls water away from yeast - leaving the yeast thirsty and unable to grow. If you're using regular yeast not formulated for sweet dough, you may find the rising time for these rolls will be longer by 30 minutes or more for each rise. This is a very wet dough, and can be tricky to handle. If you're having difficulty, we suggest lightly oiling your hands and counter before attempting to roll out the buns. If you want to use fresh pineapple juice: microwave the juice for 2 minutes until it reaches 200°F and allow to cool to lukewarm before using. The process of heating will kill any enzymes in the juice, which would otherwise destroy the gluten. To make hamburger-sized buns, prepare the dough according to the recipe instructions, and after the rise divide it into six equal pieces. Round each piece into a smooth ball, and place them into the lightly-greased wells of a hamburger bun pan, flattening them gently. Let the buns rise for an hour, until they fill the wells and are nicely puffy, then brush the rolls with the egg wash, and bake them for 20 to 25 minutes until they're a deep golden brown.