Combine warm milk, yeast, and sugar in a large bowl. Let stand for 5-10 minutes until foamy.

Add egg, melted butter, and salt to the yeast mixture. Whisk to combine. Gradually add flour, mixing until a shaggy dough forms.

Knead the dough for 5-7 minutes until smooth and elastic. Let rise in a warm place for 1-1 ½ hours, or until doubled.

While the dough rises, cream together butter, brown sugar, cocoa powder, and cinnamon for the filling.

Punch down the dough, roll it into a 12x18 inch rectangle, spread the filling evenly, and roll it into a log. Cut into 12 slices.

Place rolls in a greased 9x13 inch pan; let rise for 30-45 minutes, or until almost doubled. Preheat oven to 350°F (175°C).

Bake for 20-25 minutes, or until golden brown. Let cool for 10 minutes before frosting.

Whisk together powdered sugar, milk, and vanilla for the icing. Drizzle over warm rolls and serve.