Preheat oven to 350ºF and prepare a large muffin tin with parchment paper or muffin liners.
In the bowl of a stand mixer with the paddle attachment, beat the butter and sugar together until fluffy and fully combined.
Mix in the egg, vanilla, and mashed bananas until fully combined.
In a separate bowl, mix together the flour, baking powder, baking soda, and salt.
Mix the dry ingredients into the wet ingredients until just combined and no streaks of flour remain.
Make the streusel topping in a small bowl by hand-mixing the butter, brown sugar, flour, and crushed biscoff cookies together until it reaches a crumbly consistency.
Use a cookie scoop to scoop the batter evenly into each muffin liner, then sprinkle the streusel topping onto each muffin evenly.
Bake the muffins for about 35 minutes, or until they’ve risen and a toothpick comes out clean. Top with more Biscoff cookies if desired before eating!