In a large bowl, whisk together the flour, sugar, salt, and yeast.

Add the lukewarm milk, egg, and melted butter. Mix until a shaggy dough forms.

Turn the dough out onto a lightly floured surface and knead for 8-10 minutes until smooth and elastic.

Place the dough in a lightly oiled bowl, cover, and let it rise in a warm place for 1.5 to 2 hours, or until doubled in size.

Cut the chilled butter into 1/2-inch slices and arrange them in a 6x6 inch square between two sheets of parchment paper. Use a rolling pin to press and roll the butter into a smooth, even square.

Punch down the risen dough and roll it into a 12x12 inch square.

Place the butter block diagonally in the center of the dough square. Fold the corners of the dough over the butter to fully enclose it, pinching the seams to seal.

Roll the dough into a 16x8 inch rectangle. Perform a single turn (fold the dough into thirds like a letter). Wrap and chill for 1 hour.

Repeat the rolling and single turn process two more times, chilling for 1 hour after each turn. This completes the lamination.

Roll the laminated dough into a large, thin rectangle (about 20x12 inches).

Cut the rectangle lengthwise into 12 long strips (about 1 inch wide).

Roll each strip tightly into a spiral, then place the spiral seam-side down into a greased standard muffin tin.

Cover the cruffins loosely and let them proof for 30-45 minutes until slightly puffy.

Preheat oven to 375°F (190°C). Bake for 20-25 minutes, or until golden brown and flaky.

While the cruffins are still warm, brush them generously with the melted butter.

Immediately roll them in the cinnamon-sugar mixture until fully coated.

Serve warm, or fill with pastry cream or jam once cooled.