Make the preferment:

1. Heat the 6 tbsp milk in a small saucepan over medium-low heat until warm (80–90°F / 27–32°C). Transfer to a medium bowl and stir in the yeast. Let sit for 1 minute, then mix in the flour until smooth. Cover and let rise until doubled, about 2–3 hours at room temperature or overnight in the refrigerator.

Make the dough:

1. Place the preferment in the bowl of a stand mixer fitted with the dough hook. Add the yeast and mix on medium speed for about 4 minutes, scraping down the sides as needed.
2. Add half the milk and continue mixing on medium speed until fully incorporated. Add the remaining milk along with the flour, sugar, salt, and melted butter. Mix on low until the dough comes together in a shaggy mass, about 3 minutes. Cover and rest for 20 minutes.
3. Return to low speed and mix until the dough is smooth and elastic, about 3–4 more minutes. Cover and let rise until the volume increases by half, about 1–2 hours.
4. Dust a work surface with flour. Place the dough on top, press into a rectangle about 2 inches thick, wrap in plastic, and refrigerate for 4–6 hours.

Laminate the dough:

1. About 1 hour before laminating, beat the cold laminating butter in the stand mixer with the paddle attachment until smooth and slightly softened. Shape into a flat rectangle on a sheet of plastic wrap. Keep at room temperature if the kitchen is cool, or refrigerate briefly if warm — the butter should be pliable but not soft.
2. On a lightly floured surface, roll the dough into a 20 x 10-inch rectangle with the long side facing you. Spread the butter over the left two-thirds of the dough, leaving the right third bare.
3. Fold the bare third over the buttered center, then fold the remaining buttered third over that (like a letter). Pinch the seams to encase the butter completely. Turn the dough 90 degrees, then roll out to another 20 x 10-inch rectangle. Fold into thirds again. Wrap loosely in plastic, place on a sheet pan, and refrigerate for 1–2 hours.
4. Repeat the roll-and-fold one more time. Rewrap and refrigerate for an additional 1–2 hours, or overnight.

Assemble and bake the morning buns:

1. Mix the granulated sugar, brown sugar, orange zest, cinnamon, and salt in a small bowl until well combined.
2. Brush the cups of a 12-cup muffin tin generously with the 1 tbsp melted butter, then coat each cup with a sprinkle of granulated sugar. Line a sheet pan with parchment paper.
3. On a lightly floured surface, roll the dough out to a 16 x 12-inch rectangle. Brush the surface with the 3 tbsp melted butter, then sprinkle the sugar-cinnamon filling evenly over the top, leaving a small border.
4. Starting from the long edge nearest you, roll the dough into a tight log. Pinch the seam to seal. Transfer the log to the parchment-lined sheet pan, cover loosely with plastic wrap, and refrigerate for 30 minutes.
5. Using a sharp knife, cut the log into 12 even rounds. Place each round cut-side up in a muffin cup. Cover loosely with plastic wrap and let proof in a warm spot until the buns are puffed and filling the cups, about 1 hour.
6. Preheat the oven to 375°F (190°C). Set the muffin tin on a foil-lined sheet pan (to catch any drips) and bake until deep golden brown, 45 minutes to 1 hour.
7. Immediately dust the tops generously with granulated sugar as soon as the buns come out of the oven. Let them sit in the tin for 2–3 minutes, then gently remove and cool upside down on a wire rack. Serve warm.