Preheat oven to 425°F and grease an 8×11-inch baking dish with butter. Set aside.
In a small bowl, combine the filling ingredients: brown sugar, granulated sugar, cinnamon, salt, and melted butter. Mix until well blended and set aside.
In a large bowl, whisk together the flour, sugar, baking powder, and baking soda. In a separate container, mix together the buttermilk, 4 tablespoons of the melted butter, and vanilla extract and pour over the flour mixture. Using a wooden spoon, stir together until a scraggy mixture forms. Knead a few times with your hands until dough is smooth and divide into two equal portions.
Turn dough out onto a lightly floured surface, and press or roll out each piece into an 8×12-inch rectangle. Sprinkle the cinnamon filling evenly over each piece of dough, leaving a 1/4-inch border. Using your hands, gently pat the filling into the dough.
Starting with the longest side nearest you (12-inch edge), roll the dough into a log. Pinch the ends closed and cut each log into 8 equal rounds, for a total of 16 rolls.
Arrange the rolls in the prepared baking dish and brush tops with remaining butter. Bake until golden brown, about 20 minutes.
While rolls bake, make the icing:
In a medium bowl, mix together the cream cheese and powdered sugar. Add milk or buttermilk until desired consistency is reached.
After removing from oven, let rolls cool in the pan on wire rack for 5 minutes. Drizzle icing over cinnamon rolls and serve while warm. Enjoy!