Preheat oven to 350°F. Grease a loaf pan with butter or cooking spray. Set aside.

In a small bowl, whisk together the flour, baking soda, and salt. Then, on a plate or in a shallow bowl, mash the bananas until very smooth with a fork or potato masher. Set aside.

In a medium bowl, whisk together the melted butter, sugar, brown sugar, eggs, vanilla, and sour cream until pale yellow, about 2 minutes. Add the mashed bananas and whisk until smooth. Next, stir the flour mixture into the wet ingredients until just combined.

Next, microwave the cookie butter in a glass liquid measuring cup for 20-30 seconds, or until it’s a drizzling consistency. Pour half of the batter into the prepared loaf pan followed by half of the cookie butter. Use a skewer or chopstick to swirl the cookie butter into the batter. Don't mix! Then, repeat with the remaining batter and cookie butter. Sprinkle the Demerara sugar over the top.

Bake for 50-60 minutes or until an inserted toothpick comes out clean. Let cool for at least 15-20 minutes before serving. Top with butter or cookie butter. Enjoy!