Mix 50g of flour 7g of active dry yeast and 50g of warm water. Cover and let it sit for 15 minutes while you prepare the other ingredients.
Into the bowl of a stand mixer, add bread flour, salt and potato flour. Mix to combine.
Then add warm milk ,room temperature pineapple juice, light brown sugar, egg, and vanilla extract, along with the sponge that you made previously. Using the dough hook attachment knead the dough until it comes together.
Then add in room temperature butter, 1 tablespoon at a time making sure that each tablespoon is well incorporated before adding the next. Once all the butter has been added, mix for 5 minutes on a medium speed.
Flour a surface and remove the dough from the bowl, place it onto the floured surface and form a ball, oil the bowl and then add the dough back in, cover and let it rise in a warm place for about two hours to double in size.
Preheat the oven to 400°F.
Once the dough has doubled in size punch it down and roll it out to a 9 x 16 rectangle (measurement does not need to be exact). Starting at the short end, roll the dough up and seal the seams. Place into a greased loaf pan and cover. Let it rise for 30 to 45 minutes until just above the rim of the pan.
Mix one egg and a tablespoon of water to make egg wash. Brush with egg wash and bake at 400°F for 25 minutes or 30 minutes if you'd like the crust to be darker.