Mix the dough. Whisk the milk, yeast, egg, 1 tbsp melted butter, sugar, and salt. Stir in 3 cups flour until shaggy.
Knead and rise. Knead 3 minutes until soft and slightly tacky. Cover and rise about 1 hour, until doubled.
Make the filling. Stir the 6 tbsp softened butter, brown sugar, and cinnamon into a thick paste.
Roll and fill. Roll the dough into a 17 x 10 inch rectangle, spread the filling to the edges, and roll up lengthwise.
Cut and rise again. Use dental floss to cut 8 rolls. Place in a 9x13 pan, cover, and rise 30 minutes.
Bake. Brush with the remaining 1 tbsp melted butter and bake at 350 degrees F for 20 to 25 minutes, until golden.
Frost. Cool 15 minutes. Beat the frosting butter, cream cheese, and vanilla, add powdered sugar slowly, and spread over warm rolls.