Heat oven to 425°F with rack in upper third. Coat a 9-inch square pan with cooking spray; set aside.
In a bowl, combine the brown sugar, 2 tbsp granulated sugar and cinnamon with 2 tbsp butter; set aside.
In a large bowl, whisk together the flour, 3 tbsp granulated sugar, baking powder, baking soda and salt. In another bowl, whisk together buttermilk, egg and 3 tbsp butter. Stir together the liquid and dry ingredients; dough will look rough. Knead on a floured surface until just smooth, about 5 turns.
Pat or roll dough into a 9-by-13-inch rectangle. Brush with 2 tbsp butter. Spread dough with brown sugar filling, leaving ½-inch border. Use a spatula to lift dough from surface and roll, starting at a long side, to form a tight log. Pinch seam to seal log and slice evenly into 9 pieces. Place in prepared pan swirl-side up and press rolls down slightly. Brush tops with 1 tbsp butter.
Bake until edges are just golden brown and rolls are cooked through, 22 to 25 min. Loosen rolls from pan and invert onto a plate; re-invert to right side up onto a rack. Cool 5 min before icing.
For icing: In a bowl, beat together cream cheese and buttermilk until smooth. Beat in confectioners' sugar to form icing; spread over rolls with knife. Serve rolls warm.
Trust me: it's worth the time to make these mouthwatering rolls from scratch rather than baking the prepackaged kind.