01 - In a large mixing bowl, whisk together the flour, sugar, yeast, and salt until evenly combined. In a separate container, blend the warm milk with the egg, then pour the wet mixture 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 becomes smooth and elastic. Cover the bowl with a damp towel or plastic wrap and let the dough rise in a warm spot for 1 hour, or until doubled in bulk.
02 - Place 200 g of room-temperature butter between two sheets of parchment paper. Using a rolling pin, shape the butter into a rectangle approximately 8x10 inches. 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 roughly 16x12 inches. Unwrap the chilled butter block and lay it over one half of the dough. Fold the opposite half of dough over the butter to enclose it completely. Gently roll the dough out again to its original size, then fold it into thirds like a letter. Wrap in parchment and refrigerate for 20 minutes. Repeat this rolling and folding sequence two more times, chilling the dough for 20 minutes between each fold to keep the butter cold.
04 - Roll the laminated dough into a 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. If using a filling such as chocolate spread or jam, apply a thin, even layer across each strip.
05 - Roll each strip tightly from one end to the other, forming a compact spiral. Using a sharp knife, slice each spiral in half lengthwise to expose 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 plastic wrap or a damp towel. Allow the shaped cruffins to rise in a warm, draft-free area for 45 to 60 minutes until visibly puffy and nearly doubled.
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 appear crisp and flaky.
08 - Remove the cruffins from the oven and let them cool in the tin for 3 to 5 minutes. While still warm, brush the tops lightly with melted butter and roll in cinnamon sugar if desired. Transfer to a wire cooling rack. Serve as is, or fill with lemon curd, Nutella, or pastry cream using a piping bag for an extra indulgent presentation.