Preheat the oven to 190°C, fan 170°C, gas 5. Put the plums in a medium-sized ovenproof dish or roasting tin with 2 tbsp water. Trickle over the honey, sprinkle with cinnamon and mix well. Put the dish or tin in the oven and bake for 10 minutes until the plums have started to soften and have released their juices.
Meanwhile, in a bowl, rub the butter into the flour using your fingertips until it resembles breadcrumbs. Mix the egg and buttermilk together in a jug, then stir into the butter and flour mixture to form a soft dough.
Drop the dough, a tablespoon at a time, over the top of the plums. Scatter over the almonds and bake for 25 minutes until the topping has risen, is golden and cooked through.