Boil the Potato (1) in Water (1 cup) uncovered until soft, 12 to 15 minutes.
Drain and reserve the liquid, and let the liquid cool to 95-110 degrees F (35-45 degrees C) until lukewarm. While you are waiting, mash the potato and set aside.
Combine 2 tablespoons of the cooled liquid and the Active Dry Yeast (1 1/2 teaspoon) in a small bowl. Set aside for 5 minutes until bubbly.
Cream 6 tablespoons of the Unsalted Butter (6 tablespoon) and Granulated Sugar (1/2 cup) until fluffy in a medium bowl. Add the Salt (1/2 teaspoon) and Egg (1). Stir in the mashed potato and the yeast mixture.
Add 1/3 cup of the reserved cooking liquid and add the Unbleached All-Purpose Flour (2 1/2 cup), a little at a time, to form a dough. Turn the soft, moist dough out onto a lightly floured work surface. Knead until the dough feels smooth and elastic, about 5 minutes.
Transfer to a greased bowl, cover and set aside at room temperature for about one and a half hours, until doubled.
Grease the bottom and sides of a 9×12-inch baking sheet or jelly roll pan. Return the dough to a work surface and roll or push it out into a rectangle the size of the pan. Place the dough in the pan and push it down to fill the pan. Cover and let rise for 45 minutes.
Preheat oven to 350 degrees F (180 degrees C). Melt the remaining Unsalted Butter (2 tablespoon).
Dimple the dough by making deep indentations with your fingertips at 1 1/2-inch intervals.
Drizzle with melted butter and sprinkle Brown Sugar (1/3 cup) and Ground Cinnamon (1 1/2 teaspoon) on top.
Bake until the crust is golden brown and sugar is crunchy, about 20 to 25 minutes. Cool in the pan on a wire rack. Cut into 3-inch squares and serve warm. Enjoy!