Preheat oven to 350 degrees Fahrenheit.
Place a heavy-bottomed frying pan over medium heat, add the butter and melt it completely.
Remove the stone from each plum, then chop the plums into chunks.
Add the plums to the frying pan, sprinkle the plums with sugar, then cook for 2-3 minutes until the plums are softened.
Add the cayenne pepper and the cinnamon powder to the plum mix, stir to combine, then set the plums to the side off the heat.
To a food processor, add the amaretti biscuits, flour, baking soda, and remaining butter. Pulse blend until the mix resembles bread crumbs and until no larger lumps of butter remain.
Place the cooked plum into the bottom of small casserole dish then sprinkle the topping evenly over top.
Transfer into the pre-heated oven, and bake for 30 minutes or until the top is golden brown. Remove from the oven.
Serve with cream, custard, or ice cream!