Set the oven rack to the center position and preheat to 350°F.
Lightly grease a 9-inch by 5-inch loaf pan with cooking spray or vegetable oil.
In a small bowl combine ⅓ cup dark brown sugar and 1 tablespoon cinnamon, set aside.
Melt the butter in a pan or microwave, and then cool to room temperature.
In a large bowl whisk together flour, baking powder, cinnamon, and salt.
In a medium bowl whisk together granulated sugar, brown sugar, and eggs until smooth and pale yellow in color, 1 minute.
Slowly whisk in melted butter and vegetable oil into the egg mixture until incorporated.
Whisk the milk, sour cream, and vanilla into the egg mixture.
Add the egg mixture to the flour mixture. Gently fold the flour mixture until just moistened, with some lumps and flour spots remaining. Do not over mix the batter.
Add half of the batter to the pan.
Sprinkle half of the cinnamon and brown sugar mixture over batter.
Top with the rest of the batter, then sprinkle the rest of the cinnamon mixture on top.
Use a knife to swirl the cinnamon mixture with the batter, up and down into the batter, and around the pan in swirls, but do not completely mix.
Bake the cinnamon bread until a toothpick inserted comes out clean and the bread is set, or internal temperature reaches 205 to 210°F with a thermometer, about 45 to 55 minutes. Cover the bread loosely with foil about 30 minutes into cooking to prevent the top from getting too brown.
Cool cinnamon bread in the pan for 10 minutes. Carefully remove and transfer to a cooling rack.