1. Preheat oven to 350°F and grease a loaf pan.
2. In a mixing bowl, whisk together flour, baking powder, baking soda, salt, and sugar.
3. In another bowl, mix eggs, oil, milk or buttermilk, vanilla, and sour cream or yogurt.
4. Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
5. In a small bowl, mix cinnamon and sugar to create the cinnamon swirl mixture.
6. Pour half of the batter into the greased loaf pan.
7. Sprinkle half of the cinnamon sugar mixture over the batter.
8. Add the remaining batter on top.
9. Sprinkle the rest of the cinnamon sugar mixture and gently swirl with a knife.
10. If using a streusel topping, mix flour, sugar, cinnamon, and melted butter, then sprinkle over the top.
11. Bake for 45-55 minutes or until a toothpick inserted in the center comes out clean.
12. Let the bread cool in the pan for about 10 minutes before transferring to a wire rack.
13. Serve warm with butter or glaze if desired.
14. Store leftovers in an airtight container or freeze for later enjoyment.