Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan.
In a small bowl, mix cinnamon and sugar for the swirl.
In another bowl, whisk flour, baking powder, and salt.
Cream butter and sugar together until fluffy. Mix in egg, milk, vanilla, and oil.
Gradually add dry ingredients, mixing until just combined.
Pour half the batter into the pan, sprinkle with half the cinnamon sugar.
Add remaining batter, top with the rest of the cinnamon sugar, and swirl with a knife.
Bake for 50-60 minutes or until a toothpick inserted in the center comes out clean.
Let cool before slicing and serving. Enjoy!