Line a 20.5cm square tin with baking parchment or a double layer of clingfilm, leaving excess hanging over the sides.  Melt the white chocolate, butter and Biscoff spread in a large heatproof bowl set over a pan of barely simmering water, stirring occasionally (it will look slightly grainy because of the spread).  Remove from heat and gently stir in the biscuits and marshmallows. Scrape into the prepared tin and chill for 3hr, until set. Remove from tin and transfer to a board. Drizzle with caramel sauce, if using, and serve in squares.