Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line with parchment paper.
Step 2: In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
Step 3: In a large bowl, cream together the softened butter and 3/4 cup granulated sugar until light and fluffy (about 2-3 minutes).
Step 4: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 5: Add the mashed bananas and sour cream (if using) to the wet ingredients and mix until just combined.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined. Be careful not to overmix; a few lumps are okay.
Step 7: In a small separate bowl, prepare the cinnamon swirl mixture by combining the 1/4 cup granulated sugar and 1 tablespoon ground cinnamon.
Step 8: Pour half of the banana bread batter into the prepared loaf pan. Sprinkle half of the cinnamon sugar mixture evenly over the batter.
Step 9: Pour the remaining batter over the cinnamon sugar layer. Sprinkle the remaining cinnamon sugar mixture on top.
Step 10: Using a butter knife or skewer, gently swirl through the batter a few times to create a marbled effect. Do not overmix.
Step 11: Bake for 50-60 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean or with moist crumbs attached. If the top browns too quickly, tent loosely with foil.
Step 12: Remove the loaf from the oven and let it cool in the pan for 10-15 minutes before transferring to a wire rack to cool completely. Slice and serve.