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 ingredients. 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 and let the dough rise for 1 hour or until doubled in bulk.
02 - Place the room-temperature butter between two sheets of parchment paper and roll it into a rectangle approximately 8x10 inches. Transfer to the refrigerator and chill until the butter is firm yet still pliable, about 15 to 20 minutes.
03 - On a lightly floured surface, roll the risen dough into a large rectangle roughly 16x12 inches. Lay the chilled butter block over one half of the dough and fold the other half on top to enclose the butter. Roll the dough out gently to lengthen it, then fold it into thirds like a letter. Wrap in parchment and refrigerate for 20 minutes. Repeat this rolling and folding process two more times, chilling the dough between each fold to keep the butter layers distinct.
04 - Roll the laminated dough out once more into a 16x12-inch rectangle. Trim the edges with a sharp knife or pizza cutter for clean, even lines. Cut the dough lengthwise into 12 equal strips. If using a filling, spread a thin, even layer of chocolate spread, jam, or your chosen filling across each strip.
05 - Roll each strip tightly from one end to the other into a compact spiral. Slice each spiral in half lengthwise to expose the flaky interior layers. Twist each half into a loose knot and place it cut-side up into a generously greased 12-cavity muffin tin.
06 - Loosely cover the muffin tin with a damp cloth or plastic wrap and let the shaped cruffins rise at warm room temperature for 45 to 60 minutes until visibly puffed and nearly doubled.
07 - Preheat the oven to 400°F. 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 are crisp and flaky.
08 - Remove the cruffins from the oven and let them cool in the tin for 3 minutes. While still warm, brush the tops with melted butter and roll them in cinnamon sugar if desired. Transfer to a wire rack. Serve as is, or use a piping bag fitted with a narrow tip to inject lemon curd, Nutella, or pastry cream into the centers.