Preparation

Peel potato, and boil until soft. Drain, mash, and reserve.

Combine egg, shortening, salt, milk, 2 tsp. mashed potato, and sugar in the bowl of a stand mixer outfitted with a paddle. (If working with a hand mixer, use the whisk attachment.) Mix for 4 minutes. If you are using a stand mixer, switch to the dough hook and add bread flour and cake flour. If not, continue with your hand mixer. Combine yeast and water and stir it into the other ingredients. With a dough hook, continue mixing on medium speed for 7 minutes, or until the dough is smooth and elastic. Otherwise, turn the dough out onto a lightly floured surface and knead for about 10 minutes, until it is smooth and elastic.

Cover and let dough rise about 1½ hours, or until it has doubled in size.

Using shortening, grease a 12 x 18 sheet cake pan with 2-inch sides. Punch the dough down and roll it into a rectangle on a flat surface. Poke holes in it with a fork, and then fit it into the pan. It should touch the sides.

Let the dough rest in the pan and combine the butter, brown sugar and cinnamon in a pan on stovetop. Bring it to a boil, then remove it from the heat. Use your fingers to poke small indentations into the dough, making sure not to poke all the way through to the pan. Pour the warm topping over the cake, spreading gently with a small spatula to ensure an even distribution. Let the dough rise again until doubled in height, about half an hour.

Preheat the oven to 365 degrees, and bake the cake for 15-17 minutes, or until it is lightly browned. Check it for bubbles after about 10 minutes in the oven. If you see a bubble forming, burst it and continue baking. To serve, warm the cake and cut it into squares.