01 - In a large mixing bowl, whisk together the flour, instant yeast, sugar, and salt. Pour in the lukewarm water, milk, and melted butter, then stir until a shaggy dough forms and all dry ingredients are incorporated.
02 - Turn the dough onto a lightly floured surface and knead for 5 to 7 minutes until smooth, springy, and elastic. Shape into a ball, place in a lightly oiled bowl, cover with plastic wrap or a damp towel, and let rise in a warm spot for 1 hour or until doubled in volume.
03 - While the dough rises, place the room-temperature butter between two sheets of parchment paper. Using a rolling pin, beat and roll it into a flat rectangle approximately 6 x 8 inches. Refrigerate until needed, but remove it a few minutes before laminating so it remains pliable but not melted.
04 - On a floured work surface, roll the risen dough out into a large rectangle roughly 12 x 20 inches. Place the butter block in the center and spread it evenly across the dough surface. Fold the dough into thirds like a business letter. Rotate 90 degrees, roll it back out to the same dimensions, and fold into thirds once more.
05 - Wrap the folded dough tightly in plastic wrap and refrigerate for 1 hour to firm the butter layers and relax the gluten.
06 - Remove the dough from the refrigerator and roll it out once more into a 12 x 20 inch rectangle. Using a sharp knife or dough scraper, cut the dough lengthwise into 12 even strips approximately 1 inch wide.
07 - Working one strip at a time, gently stretch it if needed and roll it up tightly from one end to form a small log. Cut the log down the center lengthwise with a sharp knife or dough scraper to expose the interior butter layers. Twist each half into a loose spiral, then coil it snugly and place it into a greased muffin tin cavity with the layered side facing outward.
08 - Cover the muffin tin loosely with a damp towel or oiled plastic wrap. Let the shaped cruffins rest at room temperature for 45 minutes to 1 hour until visibly puffy and nearly doubled in size.
09 - Preheat the oven to 400°F. Bake the cruffins on the center rack for 18 to 22 minutes until the tops are deep golden brown and the layers have puffed and separated. Rotate the tin halfway through for even browning.
10 - Cool the cruffins in the tin for 3 to 5 minutes, then gently remove them. If using the cinnamon sugar topping, roll each warm cruffin in the mixed cinnamon sugar until evenly coated. Serve warm or at room temperature.