When I first got this recipe, I thought it would be strange to make this cake then refrigerate it and bake the next morning. But it's great. The cake is so moist and delicious. It's wonderful when you have company staying at your house. It makes a great breakfast with a cup of coffee.
Lightly grease an 8 inch square baking pan. In a large bowl, cream together the butter, white sugar, and 1/4 cup brown sugar. Beat in the egg until well blended. In a medium bowl, combine the flour, baking powder, baking soda, and 1/2 teaspoon cinnamon. Stir the flour mixture into the creamed mixture alternately with buttermilk. Spread evenly into the prepared baking pan.
In a small bowl, mix 1/4 cup brown sugar, walnuts, and 1/4 teaspoon cinnamon. Sprinkle over the batter. Cover, and refrigerate overnight.
The next day, preheat oven to 350 degrees F (175 degrees C). Bake the cake for 40 to 45 minutes in the preheated oven, or until a toothpick inserted in the center comes out clean.