Preheat your oven to 350°F (175°C) and line a 9x5 inch loaf pan with parchment paper, allowing some overhang for easy removal.
In a large bowl, whisk together the flour, baking powder, and salt.
In a separate medium bowl, whisk together the sugar, egg, milk, oil, and vanilla extract until the mixture becomes slightly frothy.
Pour the wet ingredients into the dry ingredients and stir just until combined. Be careful not to overmix.
In a small bowl, mix together the sugar and cinnamon for the swirl. Melt the butter separately.
Pour half of the batter into your prepared loaf pan. Sprinkle two-thirds of the cinnamon-sugar mixture evenly over the batter, then drizzle with half the melted butter.
Add the remaining batter on top. Use a butter knife to create 3-4 figure-eight patterns through the batter, creating beautiful swirls.
Sprinkle the remaining cinnamon-sugar on top and drizzle with the last of the melted butter.
Bake for 45-50 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs attached. Tent with foil after 30 minutes if the top is browning too quickly.
Let the bread cool in the pan for 15 minutes, then lift out using the parchment paper and cool completely on a wire rack.
If adding the optional glaze, whisk together the powdered sugar, milk, and vanilla until smooth. Drizzle over the cooled bread.
Slice and serve when completely cool.