Moravian Sugar Cake

Place potato in a small saucepan with enough water to cover it by 1 inch; bring to a boil. Reduce heat, cover, and simmer until very tender, 10 to 15 minutes.

Transfer potato to a strainer and let sit until completely drained. Force it through a ricer into a bowl. Stir in 2 tablespoons warm water.

Proof yeast in a small bowl with remaining 1/2 cup warm water until mixture is foamy, about 5 minutes.

Stir yeast mixture, mashed potato, 2 1/2 cups flour, sugar, melted butter, egg, and salt together in a large bowl until well combined.

Turn dough out onto a floured surface and knead, 8 to 10 minutes, adding as much of the remaining 1/2 cup flour as necessary to form a smooth, elastic dough. Transfer the dough to large, buttered bowl, turning to coat with the butter. Cover with plastic wrap and let rise in a warm place until doubled in size, 1 1/2 to 2 hours.

Punch dough down, then press it evenly in a buttered 9x13-inch baking pan. Cover with a kitchen towel and let rise in a warm place until puffed, 30 to 45 minutes.

Meanwhile, preheat the oven to 400 degrees F (200 degrees C).

Make indentations all over the top of the dough with your thumb and scatter the cold butter pieces over top. Stir brown sugar and cinnamon together in a small bowl and sprinkle evenly over the dough.

Bake in the middle of the preheated oven until dark brown and a toothpick inserted into the center comes out clean, 20 to 25 minutes. Let the cake cool for 5 minutes and cut it into squares.