Place the butter and sugars in the large bowl of an electric mixer and beat until well combined, 2 – 3 minutes. Add the eggs and beat until eggs are just incorporated.
Combine flour, cinnamon, baking powder, soda and salt in a small bowl. Add to batter alternately with buttermilk. Beat a minute or two at medium-low so that batter is smooth.Lightly mist and flour a 13″ x 9″ baking pan with vegetable oil and flour. Shake out the excess flour. Pour the batter into the prepared pan.
In a small bowl, combine light brown sugar, pecans and cinnamon. With your fingers, scatter this topping evenly over the batter. Dribble the topping with melted butter or margarine. Cover the pan with plastic wrap and refrigerate overnight.
The next morning, place a rack in the center of the oven and preheat the oven to 350 degrees F.
Remove the pan from the refrigerator and remove the plastic wrap. Place the pan in the oven and bake 30 – 32 minutes, or until the cake springs back in the center when lightly pressed with your finger and a toothpick inserted in the center comes out clean. Remove the pan to a wire rack to cool to 15 minutes. Cut into squares and serve warm.