Crush the biscuits for the crust. Mix with melted butter and milk. Press firmly into an 18 cm pan. Refrigerate while preparing the filling.
Briefly beat mascarpone with powdered sugar just until combined. Add cold heavy cream and mix until semi-firm. Do not overwhip. Divide the filling into two equal parts.
Mix crushed biscuits into one half. Spread over the crust.
Mix Biscoff spread into the other half until smooth. Spread over the first layer.
Refrigerate for 3 hours.
Decorate as desired (melted Biscoff spread, crushed/broken biscuits, whipped cream, etc.).