Preheat the oven to 190°C and insert a baking sheet.
Crush amaretti biscuits into coarse crumbs using a bag and rolling pin.
Melt 30g of butter in a large pan and add plums, 2 tablespoons of sugar, and lemon zest and juice.
Cook the fruit for two minutes uncovered and two minutes covered until tender.
Pour fruit into a 23x6cm deep ovenproof dish and sprinkle with 2 tablespoons of crushed amaretti.
Combine flour and baking powder in a mixer, then beat in cold butter until it resembles large oatmeal flakes.
Mix in the sugar and the remaining amaretti crumbs with a fork.
Spread the crumble mixture over the fruit, sealing the edges.
Bake on the preheated sheet for 30 minutes until bubbling and golden.
Let the crumble stand for 15 minutes before serving.