For the dough:
In a small bowl, combine the water and yeast and stir to dissolve. Let stand for 10 minutes or until foamy.
Place the yolks in the bowl of a stand mixer fitted with the paddle attachment. In a small saucepan, heat the butter and milk over medium-low heat until the butter has melted. With the mixer running on low speed, carefully pour the butter mixture into the yolks, and mix until combined. Let stand until just warm to the touch.
With the mixer on low speed, add the salt, sugar, and yeast mixture, mixing until just combined. Stop the mixer and add 2 cups of the flour, then mix on low speed until just incorporated. There will still be some lumps. Add the remaining 2 cups flour and mix for 30 seconds. Increase the speed to medium and continue to mix until the dough is smooth, about 5 minutes; it will still be sticky.
Lightly butter a large bowl. Using a plastic dough scraper, transfer the dough to the bowl. Turn to coat the dough with butter. Cover with a kitchen towel or plastic wrap and let rise in a warm place for 1 to 1 1/2 hours, or until doubled in size.
For the topping:
Place the pecans, chocolate, and sugar in a food processor and pulse until the mixture has a sandy consistency.
Set aside. Butter and sugar a 12-cup (l0-inch) Bundt pan.
Pour the melted butter into a small bowl. Pull off small pieces of the dough, and roll into balls about the size of a golf ball. Roll in the butter, then in the nut and chocolate mixture to coat, and place in the prepared pan. Line the bottom of the pan and then stack more coated balls on top until all of the dough is used. Arrange the balls (and make smaller balls if necessary) so that the top layer is fairly even. Cover with a kitchen towel or plastic wrap and let rise in a warm spot for about 1 hour, or until doubled in size. Reserve any remaining melted butter.
Position a rack in the middle of the oven and preheat the oven to 350degreesF. Brush the top of the kuchen with any remaining melted butter. Bake for 45 to 50 minutes, or until the top is dark brown and a skewer inserted in the center comes out clean. The melted chocolate and sugar will be hot, so be careful.
Let the kuchen stand for 3 to 5 minutes, then turn out onto a cooling rack and let cool for 15 minutes. Serve warm.