Preheat the Oven: Preheat the oven to 350°F (175°C). Line a 12-hole muffin tin with paper muffin liners and set aside.
Prepare the Dry Ingredients: In a large mixing bowl, add 2 cups self-raising flour and 1 cup brown sugar. Mix until well combined, then set aside.
Prepare the Wet Ingredients: In a separate bowl, mash 2 ripe bananas until smooth. Add 1 cup milk, ½ cup melted butter, ½ cup Biscoff spread, and 1 large egg. Whisk until well combined.
Combine Wet and Dry Ingredients: Add the banana mixture to the dry ingredients and stir gently until everything is just combined. Be careful not to overmix the batter.
Fill the Muffin Tin: Divide the muffin batter evenly into the muffin tin, filling each cup about ¾ full. Do not overfill.
Bake the Muffins: Bake for 20 minutes, or until the muffins are golden on top and a toothpick inserted into the center comes out clean.
Cool and Drizzle: Remove the muffins from the oven and let them cool on a wire rack for 5 minutes. While cooling, melt the remaining 2 tablespoons Biscoff spread in the microwave for about 10-20 seconds, then drizzle over the muffins once they have cooled completely.