Cook potato in lightly salted water until soft. Drain potato, mash in a bowl and let cool completely. (you can used leftover mashed potatoes, just measure the weight)
In a large bowl, mix flour, salt, potato, butter and work into a dough ball. Add egg yolk and yeast, knead into a medium soft dough, adjusting the flour as necessary. Cover and let rest in a warm place for 1 – 1 1/2 hours.
On a floured work surface, roll out dough to 1 inch (2.5 cm) thickness, and cut out biscuits with a 1 inch diameter round shape (I used a small glass). Dip the cutter in flour to avoid dough sticking to the cutter.
Place biscuits on a large cookie sheet covered with parchment, and let them rest for another 1/2 – 1 hour.
Preheat oven to 400 degrees F (200 Celsius), brush the top of the biscuits with egg white. Bake in preheated oven for 20-25 minutes or until done. Let biscuits cool on a wire rack before serving.