01 - In a large mixing bowl, whisk together the flour, sugar, yeast, and salt. In a separate container, blend the warm milk with the egg, then pour the liquid into the dry mixture. Stir until a shaggy dough begins to form, then add the softened butter. Knead by hand or with a stand mixer fitted with a dough hook for 8 to 10 minutes until the dough is smooth, elastic, and slightly tacky. Cover the bowl with a damp towel or plastic wrap and let the dough rise in a warm spot for 1 hour, or until it has doubled in volume.
02 - Place the room-temperature butter on a sheet of parchment paper and shape it into a flat rectangle approximately 8x10 inches. Fold the parchment around the butter and use a rolling pin to smooth it into an even layer. Transfer to the refrigerator and chill until firm yet still pliable, about 15 to 20 minutes.
03 - On a lightly floured surface, roll the risen dough into a large rectangle about 16x12 inches. Lay the chilled butter block over one half of the dough, then fold the other half on top to enclose the butter completely. Gently roll the dough out again to its original size and fold it into thirds like a business letter. Wrap in plastic and refrigerate for 20 minutes. Repeat this rolling and folding sequence two more times, chilling the dough for 20 minutes between each turn to keep the butter firm.
04 - After the final chill, roll the laminated dough into a large rectangle approximately 16x12 inches. Trim the edges with a sharp knife or pizza cutter for clean, even lines. Cut the dough lengthwise into 12 equal strips about 1½ inches wide.
05 - If using a filling, spread a thin, even layer of chocolate spread or fruit preserves along each strip. Roll each strip tightly into a compact spiral. Using a sharp knife, slice each spiral in half lengthwise to reveal the layered interior. Twist each half into a loose knot and place it cut-side-up into a generously greased 12-cavity muffin tin.
06 - Cover the muffin tin loosely with a damp towel or oiled plastic wrap. Allow the shaped cruffins to proof in a warm, draft-free area for 45 to 60 minutes until they appear puffy and have risen noticeably above the rim of each cavity.
07 - Preheat the oven to 400°F (200°C). Bake the cruffins on the center rack for 20 to 25 minutes, rotating the tin halfway through, until the tops are a deep golden brown and the layers have puffed apart. Remove from the oven and let them cool in the tin for 3 to 5 minutes.
08 - While the cruffins are still warm, brush the tops lightly with melted butter and roll them in the cinnamon-sugar mixture if desired. Transfer to a wire cooling rack. Serve as is, or pipe lemon curd, Nutella, or pastry cream into the centers using a piping bag fitted with a long nozzle for an extra-indulgent presentation.