1Position oven racks in center and lower 3rd of oven. Place lg roasting pan filled 1/2 way w/ water, on lower rack. Heat oven to 325. Spray a 9" springform pan w/ cooking spray. Line bottom of pan w/ cut cinnamon buns, cut sides down.
2In lg bowl, w/ mixer on med-high, beat 4 pkgs, 8 oz each cream cheese and sugar until fluffy. Beat in eggs one at a time. Add sour cream, flour & vanilla. Beat until combined. Pour over buns in pan. Bake on center rack for 1 hr. Turn oven off. Leave cheesecake in oven w/ door closed for 1 hr. Remove and cool completely.
3In bowl, w/ mixer on med-high, beat 3 oz cream cheese and butter until blended. Gradually beat in powdered sugar, then milk until fluffy. reserve.
In small microwave-safe bowl, mix brown sugar, butter and cinnamon until blended. Microwave on high 15 seconds, stirring until melted. Cool slightly. Top cooled cheesecake w/ reserved topping and drizzle brown sugar melt over topping.