Melt a knob of butter in a large frying pan, add the pears and 1 tbsp sugar, and cook for 5 mins. Tip into a bowl and leave to cool. Then add the Disaronno (if using), biscuits, cinnamon, chocolate, marzipan, and 2 tbsp sugar and mix well.
Line a tray with baking parchment. Lay a sheet of filo on it, brush with melted butter and sprinkle with a little sugar. Top with a second sheet of filo and keep going until you’ve used all the pastry. Pile the filling down one of the long sides of the pastry, then roll it up to form a sausage. Make sure it is seam-side down, then tuck the ends under tightly. Roll up in the parchment, wrap tightly in foil and freeze.
Heat oven to 200C/180C fan/gas 6. Put a tray in the oven to heat. Unwrap the strudel and place, seam-side down and still on its parchment, onto the tray. Brush with melted butter and sprinkle over 1 tbsp sugar. Bake for 35-40 mins, or until golden and crisp. Leave to cool for 10 mins, then dust with icing sugar.