Heat oven to 350°F. Lightly grease a 9" x 13" pan.
Whisk together the sugar, flour and cinnamon in a medium bowl. Stir in melted butter. Set aside.
Combine brown sugar and cinnamon in a small bowl; set aside.
In a large bowl, beat the butter, sugars, salt and vanilla until smooth. Add eggs, one at a time, beating well after each addition.
In a separate bowl, whisk together the flour and baking powder. In another bowl, whisk together the sour cream and milk.
Add the flour mixture to the butter mixture alternately with the milk/sour cream mixture, mixing until smooth.
Spread half the batter (about 3 cups) into the prepared pan. Sprinkle the filling over the batter. Spread the remaining batter over the top of the filling. Use a knife to gently swirl the filling into the batter (like a marble cake).
Sprinkle the topping over the batter. Bake the cake for 50-55 minutes or until golden brown. Remove from the oven and let cool 15 minutes before cutting and serving.