Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon until well combined.

In a large mixing bowl, cream the softened butter and sugar together until light and fluffy, about 2-3 minutes.

Beat in the eggs, one at a time, then add the vanilla extract and mix until fully combined.

Gradually add the dry mixture to the creamed mixture, alternating with the buttermilk, starting and ending with the flour mixture. Do not overmix.

In a small bowl, mix 1 tablespoon of cinnamon with 1/4 cup of sugar.

Spoon half of the cake batter into the prepared pan, then sprinkle half of the cinnamon-sugar mixture over it.

Layer with the rest of the batter and top with the remaining cinnamon-sugar mixture.

Using a knife or toothpick, gently swirl through the batter to create a marbled effect.

Place in the preheated oven and bake for about 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.