Mix yellow cake mix, eggs, oil, and sour cream by hand and pour in 13 x 9 greased baking pan. Mix brown sugar and cinnamon. Pour over cake batter. Swirl into batter with knife.
Bake at 325 degrees F for 30-40 minutes. Let cake cool 10 – 15 minutes before icing.
While the cake is cooling, beat together cream cheese, butter, confectioners' sugar, vanilla extract and salt. Slowly add milk until you reach the desired consistency. Spread frosting on warm cake before serving.