Preheat oven to 350 degrees and spray a 4½" x 8½" loaf pan with cooking spray. In a large mixing bowl or the bowl of a stand mixer, cream together butter and sugar for 5 minutes, until very light and fluffy. Add egg and beat until completely incorporated then stir in vanilla and cinnamon.

Sift flour, baking soda and salt over wet ingredients and pour in buttermilk. Fold gently just until flour and buttermilk are completely combined and no wet or dry patches remain.

In a separate small bowl, stir together the sugar and cinnamon for the swirl.

Spread about ⅔ of the batter into the prepared loaf pan and spread evenly. Sprinkle about 80% of the cinnamon sugar evenly over top then add remaining batter over cinnamon sugar. Use a butter knife to swirl the cinnamon sugar through the batter in a random pattern. Tap loaf pan on counter to settle batter into pan and remove air pockets. Sprinkle top with remaining cinnamon sugar.

Bake 45–50 minutes, or until center is firm and toothpick tester comes out clean. Remove from oven and cool in pan for 20 minutes then remove from pan and cool on a rack until slightly warm or room temperature. Slice, serve and enjoy.