Step 1

To make the dough, in the bowl of a stand mixer, whisk the eggs, buttermilk and yeast. Add both flours, the white sugar and salt; attach the dough hook and mix on low until a slightly sticky dough forms, about 5 minutes, scraping the bowl and pushing the dough off the hook once or twice. With the mixer running on medium-low, add the butter 1 piece at a time, mixing for 30 to 60 seconds before adding the next piece.

Step 2

After all the butter has been added, mix on medium-low until the dough is smooth and elastic and begins to slap the sides of the bowl, 4 to 5 minutes; it will be sticky, slightly webby and cling to the bottom of the bowl. Detach the bowl from the mixer and use a spatula to scrape and gather the dough at the center. Cover with plastic wrap and let rise at room temperature for 1 hour (the dough will not double), then refrigerate for at least 8 hours or up to 24 hours.

Step 3

When you are ready to shape the dough, remove the dough from the refrigerator and make the coating. In a small saucepan over medium, cook the butter, swirling the pan, until the milk solids at the bottom are deeply browned and the butter has a rich, nutty aroma, 4 to 6 minutes. Immediately transfer to a small microwave-safe bowl, then stir in the cinnamon and nutmeg; cool for a few minutes, then stir in the vanilla. Place the white sugar in another small bowl. Mist a 12-cup Bundt pan with cooking spray.

Step 4

Lightly flour the counter and scrape the dough out onto it. Lightly flour the dough and pat it into an 8-inch square. Using a metal bench scraper or a chef’s knife, cut the dough into quadrants and cover with a kitchen towel. Place one portion on the counter and, using your hands, roll it back and forth against the counter to stretch the dough into a 16-inch rope. Using the bench scraper or knife, cut the rope into 16 pieces; it’s fine if the pieces are slightly uneven in size. Place the pieces under the towel. Roll and cut the remaining dough portions in the same way. You will have 64 pieces of dough.

Step 5

Stir the butter mixture to remix the spices. Drop 3 or 4 pieces of dough into it. Using a fork, toss the pieces to coat, then lift one out, allowing excess butter to drip back into the bowl; drop the piece into the sugar. Using a spoon, toss in the sugar to coat, then place in the prepared Bundt pan. Butter and sugar the remaining dough in the same way, piling the pieces evenly in the pan. (If the butter mixture cools and thickens as you work, microwave it on high for a few seconds.) Cover with the kitchen towel and let rise at room temperature until the pieces are puffy, fill the pan about three-fourths and slowly spring back when poked, 45 to 50 minutes. Meanwhile, heat the oven to 350°F with a rack in the lower-middle position.

Step 6

Bake until the bread is well-risen and golden brown, 30 to 35 minutes; a thermometer inserted about 2 inches from the edge into the center of the bread should register 190°F to 195°F. Cool in the pan on a wire rack for 10 minutes. Invert the bread onto a platter and lift off the pan. Cool for at least 30 minutes; if glazing, cool until barely warm to the touch.

Step 7

To make the glaze (if using), in a small bowl, whisk the powdered sugar, buttermilk, vanilla and cinnamon. Using a pastry brush, evenly brush the glaze onto the bread. Let dry for at least 30 minutes before serving.