To assemble the buns: Transfer the dough to a lightly oiled work surface. Divide in half. (At this point, each dough half may be wrapped in plastic and frozen for up to 1 month.) Working with one half at a time, roll the dough into an 8x10-inch rectangle. Lightly brush the dough with milk, then sprinkle 3/4 cup of the crumb mixture over the dough. Take one of the long edges and fold it over the center like a letter (you will be folding the dough into thirds.) Brush the top of the folded edge with milk. Gently lift the other long edge and fold it over the first so that the edges align (again, as if you are folding a letter.) Press all four edges to seal. Repeat with second half of the dough and another 3/4 cup filling.
Meanwhile, make the filling and crumb topping: In a large mixing bowl, beat together the butter, brown sugar, cinnamon, nutmeg, salt, and vanilla until thoroughly combined. Add the all purpose flour and whole wheat flour, and mix until dry crumbs form. Do not over mix, or the crumbs will become a large mass.
To prepare dough: In the bowl of a standing mixer fitted with the paddle attachment or a large mixing bowl, stir together 2 cups all purpose flour, whole wheat flour, yeast, cinnamon, salt. Add the butter, honey, and eggs and stir briefly to roughly combine. Heat buttermilk and water to the temperature specified by the yeast manufacturer (Red Star Platinum is 120 to 130 degrees F). If the buttermilk separates, simply whisk it back together, then add to the bowl. Mix until evenly moistened, then switch to a dough hook (or turn out onto a lightly floured surface) and knead for 6 minutes by machine or 8 minutes by hand, adding the remaining all purpose flour 1 tablespoon at a time. Add only enough extra flour to prevent the dough from being excessively sticky (it should still be tacky.) Transfer dough to a large bowl coated with cooking spray. Turn to coat, then cover and let rise in a warm, draft free place until doubled in size, about 1 hour. Lightly grease a 9x13-inch pan and set aside.