Make dough: Peel, cube and boil (until soft) one small potato, weighing about 3 ounces, in just enough water to cover. Strain out the potato pieces and mash them with a fork. Set aside the potato and water to cool. Freeze any extra water in ice cube trays. Use potato water in place of the liquid in any bread recipe. It'll make the dough soft and sweet. Pour lukewarm potato water or buttermilk into a small bowl. Sprinkle yeast and pinch of sugar over milk; let stand until foamy, about 5 minutes. In a bowl, whisk together sugar, 2/3 egg, and 2/3 egg yolk. Add egg mixture to yeast mixture, and whisk to combine. In the bowl of an electric mixer fitted with the paddle attachment, combine flour and salt. Add egg mixture, and beat on low speed until almost all the flour is incorporated, about 30 seconds. Change to the dough hook. Add 2 tablespoons butter, and beat until flour mixture and butter are completely incorporated, and a smooth, soft dough that's slightly sticky when squeezed is formed, about 10 minutes. Turn dough out onto a lightly floured surface, and knead a few turns until smooth. Butter a large bowl. Place dough in bowl, and turn to coat. Cover tightly with plastic wrap. Set aside in a warm place to rise until doubled in bulk, about 2 hours. Make filling and shape dough:While the dough is rising, place chocolate and cinnamon in a large bowl. Stir to combine. Stir in the Nutella until well combined. Generously butter a 9x5x 2 3/4-inch loaf pan line with parchment paper. Beat remaining 1/3 egg with 1 teaspoon milk; set egg wash aside. Gently punch down the dough, and transfer to a clean surface. Let rest 5 minutes. On a generously floured surface, roll dough out into a 16-inch square; it should be 1/8 inch thick. Brush edges with reserved egg wash. Crumble the chocolate filling (reserve 2 tablespoons) evenly over dough, leaving a 1/4-inch border. Refresh egg wash if needed. Roll dough up tightly like a jelly roll. Pinch ends together to seal. Twist 5 or 6 turns. Brush top of roll with egg wash. Carefully crumble 2 tablespoons filling over the left half of the roll, being careful not to let mixture slide off. Fold right half of the roll over onto the coated left half. Fold ends under, and pinch to seal. Twist roll 2 turns, and fit into prepared pan and cover loosely with plastic wrap. Make streusel: In a large bowl, combine sugar, flour, salt, hazelnuts and butter. Using a fork, stir until fully combined with clumps ranging in size from crumbs to 1 inch. Squeeze the streusel in the palm of your hand so large clumps remain. Uncover the loaf and brush the top with egg wash. Sprinkle the streusel on top. It will seem like you have too much streusel, but pack it in there. The dough will expand later. Re-apply the plastic wrap and let stand until the dough reaches the top of the pan and is about doubled in volume, about 1 hour. Bake loaf: Fifteen minutes prior to baking, preheat the oven to 350°F Bake until the top is golden brown and the bottoms sound hollow when tapped (when loaf is removed from pan), about 40 minutes. If the top browns too quickly, cover the loaf with aluminum foil. Transfer to wire rack and cool for at least 30 minutes. Remove from pan. Serve slightly warm. Babka freezes well for up to 1 month.