Place 16 mini silicone muffin cases on a baking sheet.
Put the biscuits in a polythene bag and crush with a rolling pin until finely ground. Tip into a bowl and add the sugar, butter and egg, then sift in the flour and baking powder. Beat with a hand-held electric whisk until smooth and creamy. Divide among the cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10 minutes, or until risen and just firm. Leave in the cases for 2 minutes, then transfer to a wire rack to cool completely.
Make the icing by beating the icing sugar with the lemon juice to make a smooth paste. Spread a little over the cakes and sprinkle over pieces of the chopped plums and almonds. Drizzle a little more icing on top.