Line and lightly grease a 7-inch round springform pan with parchment paper, and preheat your oven to 350°F.
Mix the melted butter with the crushed Biscoff cookies, then press the mixture into the bottom of the pan. Place it in the fridge while you prepare the brownie filling.
In a saucepan, melt the butter and sugars over low heat. Add the milk, then turn off the heat and stir in the chocolate and Biscoff spread until melted and smooth. Let the mixture cool.
Sift the flour and cocoa powder into a large bowl.
Pour the wet mixture into the dry ingredients and mix until well combined.
Pour the batter over the cookie crust and bake for 20 minutes, or until the top is set.
Top the pie with marshmallows, then return it to the oven for an additional 5 minutes.
Let the pie cool for 15 minutes (or longer if you prefer a firmer filling).
Toast the marshmallows using a kitchen torch, and finish by sprinkling crushed Biscoff cookies on top before serving. Enjoy!