Preheat oven to 350F; generously grease a 9 by 5-inch loaf pan with butter.
Whisk together the flour, espresso powder (if using), baking soda, and salt in a medium bowl and set aside.
Cream together the butter and sugar, and then beat in the eggs until light and fluffy. Beat in the sour cream, banana, and vanilla.
Add the dry ingredients to the wet all at once and use a wooden spoon to stir just until incorporated, being careful not to over-mix.
Spread 1/3 of the batter on the bottom of the prepared loaf pan, and then sprinkle 1/3 of the filling on top of the batter. Repeat this 2 times, ending with a layer of the filling on top.
Bake until the loaf is golden and a toothpick inserted into the center comes out clean or with just a couple crumbs, about 55 to 60 minutes.
Cool the bread in the loaf pan on a wire rack before glazing.