Butter a large glass bowl and set it aside.
Boil potato cubes until fork tender. Drain and mash until completely smooth. You can use a food mill or a ricer for this; however, a fork will work just fine. Set aside the mashed potatoes to cool slightly.
Warm the milk until just slightly, not too hot, or the yeast will be killed. 80-90 degrees.
Combine the milk, egg, yeast, sugar, zest, potato, and 3 1/2 cups of the flour. Using the paddle attachment of your stand mixer, combine ingredients until a shaggy dough forms. if the dough seems too loose and wet, add the remaining 1/2 cup flour. Add hook attachment and knead in the mixer until smooth dough forms, about 15 minutes. Scraping down a few times to ensure now dry spots.
Transfer dough to the buttered bowl and let sit until it has doubled in size. I like to place the bowl in the oven with the oven light on. This helps the yeast, especially in the winter months. this should take about 1 1/2 hours.
While the dough is rising, butter a 9x13 in baking dish. I prefer to line mine with parchment for easy removal and clean up.