Soften the gelatine leaves in a bowl of cold water.
Simmer 250ml milk, 250ml cream, 25g sugar, and 50ml limoncello in a heavy saucepan.
Remove from heat and stir in the gelatine until dissolved.
Pour mixture into dariole moulds and refrigerate until set.
Melt butter and 50g sugar in a saucepan over gentle heat.
Add 150g chopped strawberries and lemon juice then remove from heat.
Pass the strawberry mixture through a sieve and allow to cool.
Unmould panna cotta onto plates and serve with the strawberry sauce.
Decorate each serving with a lemon slice dipped in icing sugar.