Preheat your oven to 350°F (175°C). Grease a 9×5-inch loaf pan so the bread releases easily after baking.

In a large bowl, mix the flour, sugar, buttermilk, eggs, oil, vanilla, baking powder, cinnamon, salt, and baking soda. Stir until everything blends into a smooth batter. It should take just a couple of minutes.

Pour the batter into the prepared loaf pan and even out the surface with a spatula.

In a small bowl, combine the sugar, softened butter, and cinnamon until it forms a crumbly mixture. Sprinkle this over the top of the batter.

Use a thin knife to gently draw swirls through the top so the cinnamon mixture creates a marbled look when it bakes.

Place the pan in the oven and bake for about 50 minutes, or until a toothpick inserted in the center comes out clean.

Let the loaf cool in the pan for a few minutes, then transfer it to a rack and allow it to cool completely before slicing.