In a medium bowl, whisk together the warm milk, melted butter, egg, yeast, and sugar until combined.
Add the flour and salt. Mix with a wooden spoon or dough hook until a dough ball forms, then knead for about 5 minutes. Add flour as needed until the dough is soft but not sticky.
Place the dough into a greased bowl, cover with plastic wrap, and let rise for 30 minutes or until doubled in size.
Punch down the dough, then turn it out onto a floured surface. Roll it into a circle.
Using a pizza cutter or knife, slice the dough into triangles (just like cutting a pizza).
Place a cube or two of butter near the wide end of each triangle, leaving about ½ inch of space at the edge.
Roll each triangle up into a crescent shape and transfer to a buttered or parchment-lined dish.
Cover lightly with a towel and let rise again for 20 minutes, or until puffy and doubled.
Whisk together the egg and milk, then brush the tops of the rolls.
Bake at 325°F for 24–27 minutes, until golden on top.
Brush with more melted butter and finish with flaky salt.
