Preheat oven to 350º. Lightly grease a 9 x 13 inch baking pan.
In a large mixing bowl, beat together the butter, brown sugar, eggs and vanilla until smooth. Add the flour, baking powder and salt.
Spread the batter evenly in the prepared pan. In a small bowl, combine the sugar and cinnamon. Sprinkle the mixture evenly over the batter in the baking pan. Bake for 25-30 minutes or until the surface springs back when gently pressed.