Step 1:Line a 24 cm/9" square baking tin with non-stick baking paper.Step 2:Heat the butter, chocolate and golden syrup in a pan over a low heat until melted. Remove from the heat.Step 3:Stir in the crushed biscuit pieces, nuts and marshmallows. Tip the mixture into the tin and smooth the top with a wet knife.Step 4:Melt the white chocolate and drizzle over the top of the cake to decorate.Step 5:Chill for about 2 hours until firm enough to cut.