Preheat your oven to 350°F and generously grease a 9×5-inch loaf pan with butter or non-stick baking spray.

In a large bowl, vigorously whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, ground cinnamon, and salt until completely uniform and the cinnamon is distributed evenly through the white flour matrix.

In a separate medium bowl, beat the eggs together with the vegetable oil, vanilla extract, and room-temperature buttermilk until fully emulsified. Pro Tip: Never use ice-cold buttermilk straight from the fridge. Bringing it to room temperature ensures that the natural lactic acids are fully reactive and ready to combine cleanly with the fat molecules in the oil and eggs.

Pour the liquid mixture directly into the center of the dry flour bowl.

Swap your whisk for a wide rubber spatula. Fold the mixture together gently, scraping from the bottom of the bowl upward, just until the dry streaks of flour vanish. Crucial: Quick breads do not use yeast; they rely entirely on chemical expansion. Aggressive mixing or using an electric beater will over-develop the flour's gluten networks, locking them into a tight, rubbery mesh that forces your loaf to turn out dense, heavy, and bready instead of cake-like and tender.

Pour the thick batter into your prepared loaf pan, using the spatula to smooth the top into a perfectly level surface.

Bake uncovered for 45 to 50 minutes. At the 45-minute mark, plunge a toothpick deep into the dead center of the loaf—it should emerge completely clean or with one or two tiny, dry crumbs. Let the hot bread rest inside the pan on a wire rack for exactly 10 minutes to allow the delicate crumb structure to set, then invert the pan to slide the loaf out onto the rack to cool completely before slicing.