Melt the butter in a small saucepan, then transfer to a food processor with the digestive biscuits and process to fine crumbs. Divide the mixture between 4 glasses and press into the base of each. Chill in the refrigerator.
Meanwhile, put the strawberries and caster sugar in a saucepan and cook, stirring, for 2–3 minutes, then leave to cool. In a bowl, mix together the mascarpone, cream, icing sugar and lemon rind and juice.
Fill the glasses with the mascarpone mixture and top each with the strawberries. Chill for 2–3 hours before serving.