Preheat the oven to 350°F (175°C). Grease a 9x5 inch loaf pan and line it with parchment paper, leaving some overhang on the sides.

In a large mixing bowl, beat the softened butter and brown sugar together using an electric mixer for 2 to 3 minutes until light and creamy.

Beat in the eggs one at a time, mixing well after each addition. Add the vanilla extract and mix until combined.

Add the mashed bananas and sour cream. Mix until combined. The mixture may look slightly curdled, which is normal.

In a separate bowl, whisk together the flour, baking soda, and salt.

Add the dry ingredients to the banana mixture and gently fold with a spatula until almost combined. Avoid overmixing.

Fold the chopped Biscoff cookies into the batter until evenly distributed.

Place the ½ cup of Biscoff cookie butter in a microwave-safe bowl and warm for 10 to 15 seconds until softened and easier to swirl.

Spoon half the banana bread batter into the prepared loaf pan. Dollop half the warmed Biscoff spread over the batter and gently swirl using a butter knife.

Spoon the remaining banana bread batter into the pan, followed by the remaining Biscoff spread. Gently swirl the top a few times without completely mixing the spread into the batter.

Bake for approximately 55 to 65 minutes. At the 45 minute mark, check the top of the bread for browning. If it is getting too dark, loosely cover the pan with aluminum foil and continue baking. Begin checking for doneness around 50 minutes. A skewer inserted into a section of bread without cookie butter should come out without wet batter.

Allow the banana bread to cool in the pan for 15 minutes before transferring it to a wire rack to cool completely.