Step 1In a small saucepan, heat three-fourths cup milk to scalding. Remove from the heat and let cool to warm.
Step 2Sprinkle the yeast over the warm water and stir until the yeast is completely dissolved.
Step 3In the large bowl of an electric mixer, combine the flour, one-fourth cup sugar and the salt. Work the butter in by hand or with a pastry cutter until the mixture resembles cornmeal.
Step 4Combine 3 lightly beaten egg yolks, the warm milk and the yeast mixture. Add to the flour mixture and beat at medium speed until completely blended and a soft dough forms, about one minute.
Step 5Divide the dough into two parts and shape each into a ball. Put each into a lightly buttered bowl. Cover and refrigerate overnight.
Step 6Remove the dough from the refrigerator; let it stand 30 minutes.
Step 7While the dough is standing, beat the egg whites until light and foamy. Gradually beat in the remaining 1 cup sugar until stiff peaks form. Beat in 2 teaspoons of the cinnamon. Fold in 1 cup of the chopped toasted hazelnuts.
Step 8Take one of the dough balls, shape it into an oval and, on a lightly floured board, roll it out to form a 10- by 18-inch rectangle.
Step 9Spread half of the meringue mixture onto the rolled-out dough, leaving a 1-inch border. Roll up the dough lengthwise jellyroll style. Pinch the ends of the roll to seal.
Step 10Gently lift the roll onto a greased large baking sheet, seam side down, and form it into a horseshoe shape. Cut slashes halfway through dough at 2-inch intervals.
Step 11Repeat the process with the remaining half of the dough.
Step 12Invert a large bowl over each of the coffeecakes. Set aside to rise about 45 minutes or until doubled.
Step 13Heat the oven to 350 degrees.
Step 14Lightly beat 1 egg yolk and 2 tablespoons milk. Brush each of the coffeecakes with egg wash, brushing the entire surface of the dough, but do not brush the meringue filling that shows.
Step 15Bake 30 to 35 minutes, until golden brown, turning the baking sheet halfway through. Cool on the pan for about 5 minutes before glazing.
Step 16For the glaze, stir together the 1 tablespoon cinnamon, powdered sugar, the remaining 2 tablespoons milk, the rum, if using, and the vanilla. Drizzle it over the coffeecakes. Sprinkle the cakes with the remaining one-half cup of chopped nuts.
Orange: Follow the master recipe, stirring 2 teaspoons grated orange peel into the filling. Replace 1 cup of the hazelnuts with toasted slivered almonds. For the glaze, substitute 2 tablespoons orange juice for the milk and stir in 1 teaspoon grated orange peel. After glazing, warm the coffeecakes and sprinkle 1 to 2 tablespoons chopped candied orange peel over the top of each. Omit the nuts on top.
Pecan: Follow the master recipe, replacing hazelnuts with toasted pecans in the filling. For the glaze, heat one-fourth cup butter over low heat until nut-brown, about 5 to 6 minutes. Remove from heat and let cool. Combine three-fourths cup powdered sugar, one-fourth cup firmly packed light brown sugar, the browned butter, 2 tablespoons milk, 2 tablespoons maple syrup and one-fourth teaspoon vanilla. Drizzle the glaze over warm coffeecakes, warming it in the microwave if necessary to maintain drizzling consistency. Sprinkle with one-fourth cup toasted pecan pieces.
Chocolate: Follow the master recipe, stirring one-fourth cup chopped semisweet or bittersweet chocolate into the filling along with chopped hazelnuts. Omit cinnamon. Sprinkle chopped hazelnuts and one-fourth cup chopped chocolate over the warm glazed coffeecakes.