Preheat oven to 350degreesF. In a large mixing bowl, beat together butter, 1 cup sugar, and egg. Combine the flour, baking powder, baking soda, and salt. Add the flour mixture to the egg mixture alternately with the buttermilk. In a small bowl, combine the cinnamon and remaining 1/3 cup sugar. Pour a third of the batter into a greased 8X4X2-inch loaf pan. Sprinkle a third of the cinnamon sugar on top of batter. Repeat layers twice. Bake for 45-50 minutes, until a toothpick inserted near the center comes out clean. Cool in pan for 10 minutes. Remove from pan to a wire rack to cool completely.