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. Then, heat the buttermilk and water to the temperature specified by the yeast manufacturer (usually 115 to 130 degrees F). If the buttermilk separates, simply whisk it back together.
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 it aside.
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.
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 the second half of the dough and another 3/4 cup filling.
Gently pat and stretch each length of dough until it is 12-inches long and even in diameter. Cut each piece into 6, 2-inch slices. Lift each slice and pinch the cut edges to seal in the filling, forming a “bun.” Some topping may fall out, but simply poke what you can back in and save any excess for sprinkling. Do not worry if the buns do not completely seal.
Space the slices evenly in the prepared pan and sprinkle with the remaining topping (much of the topping will fall into the pan.) Cover the pan with lightly greased plastic wrap, then place in a warm, draft-free place to rise for 1 hour. (The rolls will only puff slightly.) Place a rack in the center of the oven and preheat the oven to 350 degrees F.
Bake buns until the crust and crumbs are golden and the crumbs in the bottom of the pan are bubbly, 25 to 30 minutes. Remove from oven and let cool in pan for 10 minutes. Gently lift the rolls from the pan and transfer to a rack to cool. Serve warm or at room temperature with any excess pan crumbs scraped out of the pan.
Notes
TO STORE: Store leftover double stuffed cinnamon crumb buns at room temperature for up to two days.
TO MAKE AHEAD: After the first rise, you may wrap the dough tightly in plastic and freeze it for up to 1 month. Defrost the dough overnight in the fridge then finish preparing as directed. I like to bake half right away and save half for another day.