Prepare pan with baking spray containing flour. I use Wilton's Bake Easy non-stick spray for the best results.
Preheat oven to 350 degrees F.
Combine flour and baking soda in a small bowl and set aside.
In medium-large bowl, cream butter and sugar until light and fluffy (at least a few minutes).
Mix in sour cream.
Add eggs individually, mixing after each addition.
Mix in vanilla extract.
Add flour mixture alternating with milk, until blended.
Pour enough batter to cover the design (about 1/3 of batter). Over a towel covered counter, tap pan gently to release air bubbles.
Sprinkle a third of the cinnamon sugar mixture to the top of the batter layer.
Add another 1/3 batter and sprinkle with another 1/3 of cinnamon sugar mixture.
Pour remaining batter later, top off with the rest of the cinnamon sugar mixture.
Using a knife, cut the batter side-to-side to swirl the mixture throughout the batter.
Bake loaf for 45-48 minutes or until toothpick inserted in center comes out clean.
Cool cake in pan for 20 minutes and then invert onto cooling rack.
If desired, garnish with additional cinnamon-sugar.