Line an 18cm x 18cm cake tin with clingfilm. Put the chocolate in a large heatproof bowl and place over a pan of simmering water. Stir until the chocolate is completely melted.
Remove the bowl from the heat and stir in the broken digestives, mini marshmallows and most of the mini eggs.
Pour the chocolate mixture into the prepared tin and smooth the top with a butter knife. Scatter over the reserved mini eggs, pressing them in with the back of a spoon. Chill in the fridge for 2-3 hours. When set, cut into 25 small squares.