Mix the 1/3 cup sugar and cinnamon together in a small bowl and set aside.
Cream the softened butter and 1 cup of sugar together until fluffy, then beat in the egg.
Combine the dry ingredients (flour, baking soda, salt) and add them to the butter mixture alternately with the buttermilk.
Layer half the batter in a greased loaf pan, top with most of the cinnamon sugar, add the remaining batter, and swirl gently with a knife.
Top with the last bit of cinnamon sugar and bake at 350 degrees Fahrenheit for 45 to 50 minutes.