Preheat your oven to 350°F (180°C). Line and lightly grease a 7-inch (18cm) round springform pan with baking paper.
Mix the melted butter into the crushed biscuits and press the mixture into the bottom of the pan. Chill in the fridge while you prepare the brownie.
In a saucepan, melt the butter and sugar over low heat. Add the milk, then turn off the heat and add the chocolate and Biscoff spread, letting them melt. Stir until smooth and let cool.
Sift the flour and cocoa powder into a large bowl.
Pour the wet ingredients into the dry ingredients and mix until combined.
Pour the batter over the biscuit base and bake for 20 minutes, or until the top is set.
Top with marshmallows and return to the oven for 5 minutes.
Let cool for 15 minutes (or longer for a firmer filling).
Toast the marshmallows with a kitchen torch and sprinkle with crushed biscuits before serving.