Preheat the oven to 400 degrees F.
For the Brioche: Combine the yeast and milk in a small bowl and stir to dissolve the yeast. Add 1 cup flour and mix to blend well. Using a knife, scrape the vanilla bean and stir the pulp into the yeast mixture. Let sit at room temperature in a warm, draft-free place for about 2 hours to allow fermentation.
Put 2 cups of the flour into a large mixing bowl. Add 4 of the eggs, one at a time, beating thoroughly into the flour using a wooden spoon with each addition. The dough will be sticky, thick, and spongy. Add the water, sugar, and salt and mix well, beating vigorously. Add 3/4 pound butter and work it into the dough with your hands until it is well blended. Add the remaining 2 eggs and mix well into the dough. dd the remaining 2 cups of flour and blend into the dough, breaking up any lumps with your fingers. Add the yeast mixture. Using your hands, knead and fold the starter into the dough. Continue kneading and folding until all is well mixed, about 5 minutes. The dough will be sticky and moist. Cover with a clean cloth and let rise in a warm, draft-free place until it doubles in size, about 2 hours.
With your fingers, lightly punch down the dough. Cover and let rise in a warm, draft-free place until it doubles in size, about 1 hour.
To assemble: Divide the dough into 2 equal portions. Refrigerate one ball of dough while working with the other. Lightly dust the work surface with flour and roll out the dough into a rectangle, about 14 inches long and 1/4-inch thick. Using a pastry brush, egg wash the edges of the dough. In a small bowl, combine the sugar and cinnamon. Mix well. Sprinkle half of the mixture over 3/4 of the dough. Roll the dough up tightly to form a log. Wrap the log in plastic wrap and freeze for 30 minutes. Repeat the above process with the other ball of dough. In a mixing bowl, combine the butter, brown sugar and remaining 1/2 teaspoon of cinnamon. Mix well. Smear the bottom of 2 (10-inch) round cake pan with the butter mixture. Sprinkle the butter with the pecan pieces. Remove the logs from the refrigerator. Using a sharp knife, slice into 1 1/2-inch slices. Place the slices on top of the pecans, about 1/2-inch away from each other. Let the buns rise until double in size, about 1 1/2 hours. Place in the oven and bake until golden brown, about 35 minutes. Remove from the oven and turn onto a serving platter. Cool for about 5 minutes and serve warm.