Preheat your oven to 350°F (175°C). Grease a loaf pan.
In a mixing bowl, cream together the softened butter, sugar, vanilla extract, and egg until light and fluffy.
Add the buttermilk (or milk mixed with vinegar or lemon juice) to the mixture and mix until well combined.
Gradually add the flour and baking soda to the wet ingredients, mixing until smooth and well incorporated.
Pour half of the batter into the greased loaf pan.
In a separate bowl, combine the sugar and cinnamon to create the cinnamon/sugar mixture.
Sprinkle 3/4 of the cinnamon/sugar mixture over the batter in the loaf pan.
Pour the remaining batter over the cinnamon/sugar layer.
Sprinkle the remaining cinnamon/sugar mixture over the top of the batter.
Use a knife to swirl the batter and cinnamon/sugar mixture together, creating a marbled effect.
Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for about 20 minutes before removing and slicing.