In a microwave or on the stovetop, melt the butter and dark chocolate together until smooth.
Stir in the sweetened condensed milk until well combined with the melted chocolate mixture.
Crush the digestive biscuits into small pieces. You can do this by placing them in a plastic bag and using a rolling pin.
Add the crushed biscuits to the chocolate mixture. If desired, add chopped nuts and dried fruits for extra flavor and texture.
Mix all the ingredients until the biscuits and optional add-ins are evenly coated with the chocolate mixture.
Transfer the mixture into a lined or greased square or rectangular dish, shaping it into a flat cake.
Place the dish in the refrigerator and let the Lazy Cake chill for at least 2-3 hours, or until it's firm and set.