Line a 23cm (9 in) square cake tin with baking parchment.
Put the digestive biscuits into a freezer bag and seal. Tap with a rolling pin until the biscuits break into lumps and crumbs.
Combine the butter, chocolate and golden syrup into a saucepan over a low heat and stir until melted and lump free. Stir the crushed digestives, marshmallows and any other optional ingredients into the chocolate. Transfer the mixture to the prepared baking tin and be sure to spread the mixture into the corners.
Chill in the fridge for 2 hours until firm. Remove from the fridge and lift out of the tin, using the baking parchment to help you. Cut into slices as desired and serve.
If you added anything extra please leave in the comments section.