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1
In a small saucepan combine the potato, peeled and cut into 1-inch pieces, with enough water to cover it by 1 inch and simmer it, covered, for 10 to 15 minutes, or until it is very tender.
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2
Drain the potato well, force it through a ricer into a bowl, and stir in 2 tablespoons of the water.
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3
In a small bowl proof the yeast in the remaining 1/2 cup water for 5 minutes, or until the mixture is foamy.
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4
In a large bowl stir together well the yeast mixture, the mashed potato, the sugar, the butter, the egg, and the salt, add 2 1/2 cups of the flour, and stir the dough until it is combined well.
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5
Turn the dough out onto a floured surface and knead it for 8 to 10 minutes, adding as much of the remaining 1/2 cup flour as is necessary to form a smooth and elastic dough.
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6
Transfer the dough to a buttered large bowl, turning it to coat it with the butter, let it rise, covered with plastic wrap, in a warm place for 1 1/2 to 2 hours, or until it is double in bulk, and punch it down.
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7
The dough may be made 1 day in advance and kept covered and chilled.
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8
Bring the dough to room temperature before continuing with the recipe.
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9
Preheat the oven to 400F.
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10
Press the dough evenly in a buttered 13- by 9-inch baking pan and let it rise, covered with a kitchen towel, in a warm place for 30 to 45 minutes, or until it is puffed.
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11
Make indentations all over the top of the dough with your thumb and scatter the butter over the dough.
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12
In a small bowl stir together the brown sugar and the cinnamon and sprinkle the mixture evenly over the dough.
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13
Bake the cake in the middle of the oven for 20 to 25 minutes, or until it is dark brown and cooked through.
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14
Let the cake cool for 5 minutes and cut it into squares.
