Absolutely! Graham crackers or digestive biscuits are great substitutes. Add (10g) 2 tbsp of cocoa powder when making the base so that you can still get that chocolate flavour. Also, add 50g (1/4 cup) light brown sugar for sweetness. If you don't want a chocolate biscuit base then don't add in the cocoa powder and reduce the butter by 15g (a tablespoon) so that the base isn't too greasy.