Bring a medium pot of salted water to a boil over high heat. Meanwhile, peel the potatoes and cut them into 1-inch pieces. Boil them until soft when pierced with a fork, about 12 minutes. Reserve 1/2 cup of the potato cooking water in a small bowl. Drain the potatoes and cool for 5 minutes. Press them through a potato ricer into a large bowl. Set aside to cool for at least 10 minutes.
Allow the reserved potato water to cool to between 115 degrees F and 105 degrees F. Sprinkle the yeast into it, stir to dissolve, then proof for 10 minutes, or until the yeast is bubbly.
Stir the granulated sugar and 1 cup of the flour into the riced potatoes, then add the proofed yeast and its potato water. Stir just until combined, cover the bowl with plastic wrap, and set aside in a warm, dry place to double in bulk, about 30 minutes.
Stir down the sponge mixture with a wooden spoon, then stir in 2-1/2 cups flour, the eggs, butter, and salt. Turn the dough out onto a lightly floured work surface and knead by pressing into it with the heel of one hand while pulling it with the other. Knead for 8 minutes, or until the dough becomes smooth and elastic. You may need to add more flour, but be sparing and add just enough to keep the dough from sticking. Form the dough into a ball. (Alternatively, you can knead the dough in a stand mixer fitted with a dough hook. Place the dough in the bowl, then knead at medium speed for 5 minutes.)
Spray a large clean bowl with nonstick spray, place the kneaded dough in the bowl, and flip it over to coat it with the spray. Cover the bowl with plastic wrap and set aside in a warm, dry place to double in bulk, about an hour.
Meanwhile, make the filling. Toast the pecans in a medium skillet set over low heat. Toss and stir for 4 minutes, until fragrant. Place them in a medium bowl and mix in the raisins , brown sugar, and cinnamon with a wooden spoon. Stir in the melted butter until the mixture is moist and dense; set aside. Line a large baking sheet with a silicon baking mat or parchment paper.
When the dough has doubled in bulk, punch it down by gently pushing your fist into its center. Turn it out onto a lightly floured work surface and roll it out to a 12 x 20-inch rectangle. Spread the pecan filling evenly over the dough, leaving a 1/2-inch border along the long sides. Beginning with one of the long sides, tightly roll the dough into a tube, then bend the tube until its ends meet to create a ring. Place the ring on the prepared baking sheet, seam side down. Cover loosely with a clean kitchen towel and set in a warm, dry place to double in bulk, about an hour.
Meanwhile, position the rack in the center of the oven and preheat the oven to 350 degrees F. For the topping make a wash by lightly whisking the egg and water in a small bowl.
After the ring has doubled in bulk, brush the egg wash over its top, then sprinkle the ring with the sugar. With a sharp knife, make six evenly spaced, diagonal slashes across the top of the ring.
Bake for 40 minutes, or until golden. The cinnamon filling may bubble out of the slashes on the top. Cool on a wire rack for 10 minutes, then serve; or cool completely and wrap in wax paper to store at room temperature for up to two days. The ring may also be frozen, sealed tightly in plastic wrap, for up to four months.
Apricot Potato Cake: Substitute 1 cup chopped dried apricots for the raisins. Substitute 1 cup pistachios for the pecans.
Cherry Potato Coffee Cake: Substitute 1 cup dried cherries for the raisins. Substitute 1 cup sliced almonds for the pecans.
Chocolate Potato Coffee Cake: Substitute 1 cup semisweet chocolate chips for the raisins in the filling.
Cranberry Potato Coffee Cake: Substitute 1 cup dried cranberries for the raisins. Add 1 teaspoon ground ginger to the pecan filling.
Prune Armagnac Potato Coffee Cake: Omit the raisins. Soak 1 cup chopped pitted prunes in 2 Tablespoons Armagnac (brandy / cognac) for at least 4 hours or overnight. Drain, reserving the prunes and Armagnac separately. Add the Armagnac along with the other ingredients to the dough after it has risen the first time. Add the chopped prunes to the filling in place of the raisins.
Rum Raisin Potato Coffee Cake: Soak the raisins in 2 Tablespoons spiced rum for at least 4 hours or overnight. Drain, reserving the raisins and rum separately. Add the rum along with the other ingredients to the dough after it has risen the first time. Add the raisins to the filling as indicated in the recipe.
Yield: 12 servings Recipe Source: The Ultimate Potato Book by Bruce Weinstein and Mark Scarbrough (Wm Morrow)
Reprinted with permission.