Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line with parchment paper leaving an overhang on the long sides for easy removal.

In a medium bowl, whisk together the all-purpose flour, baking soda, ground cinnamon (if using), and salt. Set aside.

In a large bowl, mash the ripe bananas with a fork or potato masher until mostly smooth with a few small lumps remaining.

Add the granulated sugar, brown sugar, melted butter, egg, and vanilla extract to the mashed bananas. Whisk until well combined.

Pour the dry ingredients into the wet ingredients. Using a rubber spatula, gently fold until just combined. Be careful not to overmix; a few lumps of flour are okay.

Slightly warm the Biscoff cookie butter in a small microwave-safe bowl for 15-20 seconds, or until it's a pourable consistency. This makes it easier to swirl.

Pour about half of the banana bread batter into the prepared loaf pan. Dollop half of the warmed Biscoff cookie butter over the batter. Use a knife or skewer to gently swirl the cookie butter into the batter.

Carefully spoon the remaining banana bread batter over the Biscoff swirl. Dollop the remaining Biscoff cookie butter on top and swirl again. If using, sprinkle the crushed Biscoff cookies evenly over the top.

Bake for 55-70 minutes, or until a wooden skewer or toothpick inserted into the center comes out with moist crumbs, but no wet batter. If the top starts to brown too quickly, you can tent it loosely with aluminum foil for the last 15-20 minutes of baking.

Let the banana bread cool in the pan for 10-15 minutes before carefully transferring it to a wire rack to cool completely. Slicing when warm is tempting, but waiting until it's fully cooled will result in cleaner slices and a better set texture.