Preheat oven to 350 degrees and grease a loaf pan.
In a large bowl, whisk together the butter and sugars together. Add in the eggs and whisk until combined. Stir in vanilla, mashed bananas, milk, and sour cream.
In a separate bowl, sift together the whole wheat flour, baking powder, baking soda, and cinnamon. Add the dry ingredients to the wet and stir until just combined.
Pour batter into a greased loaf pan and bake for 1 hour to 1 hour 10 minutes at 350 degrees until toothpick inserted in the center comes out clean.
Let bread cool in the pan for approx 10 minutes, then turn out onto a rack and cool completely.