Preheat your oven to 350°F and grease a 9x5-inch loaf pan. Or line it with parchment if you’re feeling fancy.
In a medium bowl, mix together flour, baking powder, baking soda, and salt. Set it aside like it's time to shine... later.
In a large bowl, whisk together the sugar, egg, buttermilk, and oil until smooth. Don’t overthink it—just get it looking like a pourable smoothie.
Gently stir in the dry ingredients until just combined. No one wants overmixed bread—it turns moody.
Pour half of the batter into the prepared pan. Sprinkle half the cinnamon-sugar mixture over the top like fairy dust.
Add the remaining batter and sprinkle the rest of the cinnamon-sugar swirl. Use a butter knife to gently swirl through the batter for that iconic cinnamon ribbon.
Bake for 45–55 minutes, or until a toothpick comes out mostly clean (a little gooey cinnamon is expected).
Cool in the pan for 10 minutes, then move to a wire rack and try—try!—not to cut into it too soon.