Begin by preheating the oven to 350 degrees F. Then spray the inside of a 9x5-inch loaf pan with baking spray. Set aside.
In a small bowl, whisk together 1/3 cup of sugar, chopped walnuts*, and the cinnamon. Set aside.
In a large bowl, whisk together the flour, the remaining 1 cup sugar, baking powder, and salt. In a medium bowl, whisk together the egg, milk, vegetable oil, sour cream, and vanilla extract.
Add the wet ingredients to the dry and mix until just combined. (The batter may be slightly lumpy. This is okay.)
Add half of the batter to your prepared pan. Sprinkle that batter with about half of the cinnamon, sugar, and nut mixture. Top that with the remaining batter and then top with the rest of the cinnamon, sugar, and nut mixture.
Use a butterknife to make one quick swirl through the batter to give your bread a marbled look.
Bake for 60 to 67 minutes or until a toothpick inserted near the center comes out clean.** Allow the bread to cool completely before slicing to serve.