Preheat oven to 350 degrees F. Beat together butter, 1 cup of sugar, and egg until blended. Combine the flour, baking powder, baking soda, salt, and 1/2 teaspoon cinnamon. Add to egg mixture alternately with buttermilk, just until mixed.
In a small bowl, mix together remaining 1/3 cup sugar and cinnamon. Pour 1/3 of the batter into a greased 8?x4? loaf pan. Top with 1/3 of the cinnamon sugar. Repeat layers twice. Bake at 350 degrees F for 45 to 50 minutes, or until a toothpick inserted near the center comes out clean. Cool for 10 minutes in the loaf pan before removing to a wire rack to cool completely.