01 - In a large mixing bowl, whisk together the flour, instant yeast, sugar, and salt. Pour in the lukewarm water, milk, and melted butter. Stir with a wooden spoon until a shaggy dough comes together.
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, 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 roughly 6 x 8 inch rectangle of even thickness. Refrigerate until needed, but do not let it become rock-hard; it should remain pliable.
04 - On a floured work surface, roll the risen dough into a large rectangle approximately 12 x 20 inches. Place the butter block on one half of the dough, then fold the other half over to enclose it. Roll out again to 12 x 20 inches. Fold the dough into thirds like a business letter. Rotate 90 degrees, roll back out to 12 x 20 inches, and fold into thirds once more. Wrap in plastic and refrigerate for 1 hour.
05 - 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.
06 - Take one strip at a time and gently stretch it if needed. Roll it up tightly from end to end, then slice it down the center lengthwise to expose the laminated layers. Twist each half into a loose spiral and coil it into a greased muffin tin cavity, with the cut layers facing outward. Repeat with remaining strips.
07 - Cover the filled muffin tin loosely with a clean kitchen towel and let the cruffins rest at room temperature for 45 minutes to 1 hour until noticeably puffy and nearly doubled.
08 - Preheat the oven to 400°F. Bake the cruffins on the center rack for 18 to 22 minutes until deeply golden brown and well risen. Rotate the tin halfway through for even coloring.
09 - Allow the cruffins to cool in the tin for 3 to 4 minutes. If using the topping, combine the sugar and cinnamon in a shallow bowl and roll each warm cruffin in the mixture until evenly coated. Serve warm or at room temperature.