Preheat the oven to 350 degrees F and grease a metal loaf pan with nonstick spray or line with parchment paper.
In a medium bowl, whisk together the flour, sugar, baking powder, and salt. Set aside.
In a large bowl, whisk together the egg, milk, vanilla, and sour cream. Add the flour mixture to the egg mixture, stirring with a rubber spatula just until combined. Pour the batter into the prepared loaf pan.
In a small bowl, stir the brown sugar and cinnamon together and sprinkle it over the top of the batter in the pan. Run a knife through the batter to swirl the cinnamon-sugar mixture into the bread.
Bake for 45 to 50 minutes or until a toothpick inserted into the center comes out clean.
Remove from the oven and allow to cool for 15 minutes in the pan on a wire rack. Then remove bread from the pan and cool completely on the rack.
How to Make Vanilla Icing: In a medium bowl, whisk together 1 cup of powdered sugar with 2 tablespoons milk, 2 tablespoons melted butter, and 3/4 teaspoon vanilla until smooth.Nutrition is for 1 slice from a loaf cut into 8 slices.