On a small plate or in a small bowl, mash the banana until no chunks remain.
Add the biscoff cookies to a small ziploc bag and mash them up into crumbs (Alternatively, you can add them to a food processor and pulse them into crumbs.) Set aside.
Add the butter and brown sugar to a medium sized mixing bowl. Use a hand mixer or a stand mixer fitted with the paddle attachment to beat the mixture until it's light and fluffy (2-3 min.)
Add in the banana and vanilla and beat until well combined.
Add in the flour, baking powder, and salt. Gently mix just until only a few streaks of flour remain. Then, add in the biscoff crumbs and mix just until the last streak of flour disappears.
Cover the dough and leave it in the fridge to chill for at least 30 minutes, or for up to 2 days.*
Once you're ready to bake the cookies, preheat the oven to 350℉ / 175℃.
Use a 3 tbsp. cookie scoop to dish out the cookies onto the prepared cookie sheet. Evenly space the cookies apart to allow room for spreading. Bake for 10-12 minutes, until the sides of the cookies start to turn golden brown.
Immediately after taking the cookies out of the oven, take a bowl that is slightly larger than the cookies and swirl it around over the cookies to make them a little more uniform and circular.