Preheat oven to 350°F (175°C) and lightly coat a 9x5-inch loaf pan with nonstick spray or oil. In a small bowl, combine 1/3 cup of the sugar with the cinnamon; set this mixture aside. In a large mixing bowl, whisk together the flour, remaining 1 cup sugar, baking powder, and salt. In a separate bowl, blend the egg, milk, and vegetable oil, then pour the wet mixture into the dry ingredients, stirring gently until the batter is just combined. Transfer half of the batter into the prepared pan, then sprinkle half of the cinnamon-sugar blend over it. Pour in the remaining batter and top with the remaining cinnamon-sugar mixture. Using a knife, carefully swirl through the batter to create a marbled effect. Bake for 45 to 50 minutes, or until a toothpick inserted into the center emerges clean. Allow the bread to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely. For best flavor and texture, wrap the loaf in foil and let it rest overnight before slicing.