In a large bowl, combine warm milk, 1 tbsp sugar, and yeast. Let it sit for 5-10 minutes until foamy.

Whisk in the remaining sugar, egg, and melted butter. Gradually add the flour and salt, mixing until a shaggy dough forms. Knead on a lightly floured surface for 8-10 minutes until smooth. Place in an oiled bowl, cover, and let rise for 1-1.5 hours until doubled.

Punch down the dough and roll it into a rectangle. Place the cold butter block in the center and fold the dough over it like a letter. Roll it out into a long rectangle and perform another letter fold. Wrap in plastic and chill for 30 minutes. Repeat this roll-fold-chill process two more times.

Roll the chilled dough into a long, thin rectangle. Roll it up tightly into a log and cut into 12 equal pieces. Cut each piece in half lengthwise, leaving one end connected. Twist the two strands together, roll into a spiral, and place in a greased muffin tin.

Cover the muffin tin and let the cruffins proof for 45-60 minutes until puffy. Preheat oven to 400°F (200°C). Bake for 15-20 minutes until deep golden brown.

Let cruffins cool slightly. While warm, brush with melted butter. Mix cinnamon and sugar, and roll each cruffin in the mixture. Once cooled a bit more, use a piping bag to fill the center with dulce de leche.