Moravian Sugar Cake

Ingredients

• For the cake:
• 2 teaspoons instant yeast
• 1/4 cup warm water
• 1/2 cup, plus 1 teaspoon sugar
• 3 1/2 cups all-purpose flour
• 1/2 teaspoon salt
• 12 tablespoons unsalted butter, softened
• 1 egg
• 1/2 cup cooked mashed potatoes, unseasoned, room temperature
• 1/2 cup whole milk
• 2 teaspoons vanilla
• 3/4 cup light brown sugar
• 2 teaspoons ground cinnamon

Directions

1. Stir the yeast into the water until it dissolves. Scatter the teaspoon of sugar over the top, cover the bowl with plastic wrap, and leave it for 5 minutes, until the mixture looks bubbly and active. Whisk the flour with the salt in a medium bowl, then put it aside. In a stand mixer fitted with the paddle attachment, cream 8 tablespoons (1 stick) butter with the sugar until pale and soft. Beat in the egg just until it disappears, then mix in the mashed potatoes, milk, dissolved yeast, and vanilla until everything is smooth. Add the flour mixture gradually, stirring until you have a soft dough. Gather the dough into a ball and transfer it to a large greased bowl. Cover with plastic wrap and refrigerate for its rise; by morning, it should be twice as large.
2. Press down the risen dough, then turn it onto a well-floured surface and knead it gently. Roll it out to fit a 13- by 9- by 2-inch greased baking dish. Set the dough in the pan, cover it with a thin towel, and place it somewhere warm until it has doubled again, about 1 hour.
3. Set a rack in the middle of the oven and heat the oven to 375 degrees F. Put the remaining 4 tablespoons butter, brown sugar, and cinnamon in a food processor and pulse until the mixture is evenly blended.
4. With your index finger, make deep holes all over the top of the cake, spacing them evenly. Twist your finger a bit in each one to make room for the topping, then spoon the cinnamon-sugar mixture into the holes. Bake until the top is golden and the center is cooked through, about 30 to 35 minutes. Serve warm.