Activate yeast: In a large bowl, combine warm milk, yeast, and sugar. Let stand for 5-10 minutes until foamy.

Combine wet ingredients: Add the egg, melted butter, and salt to the yeast mixture. Stir to combine.

Add flour: Gradually add flour, mixing until a soft dough forms. Knead for 5-7 minutes until smooth and elastic. Add more flour if needed.

First rise: Place dough in a lightly oiled bowl, cover, and let rise in a warm place for 1-1 ½ hours, or until doubled.

Prepare filling: Combine softened butter, brown sugar, cocoa powder, and cinnamon in a bowl. Mix well.

Roll and fill: Punch down dough and roll into a 12x18 inch rectangle. Spread filling evenly over dough. Roll tightly into a log and slice into 12 rolls.

Second rise and bake: Arrange rolls in a greased 9x13 inch pan. Cover and let rise for 30-45 minutes. Preheat oven to 350°F (175°C). Bake for 20-25 minutes, or until golden brown.

Make icing: Whisk together powdered sugar, milk, and vanilla extract until smooth. Add more milk if needed.

Frost and serve: Let rolls cool slightly before frosting with icing. Serve warm.