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Heat oven to 200C/180C fan/gas 6. Mix plums, sugar, 1 tbsp flour and cinnamon in a large baking dish. Splash over the orange juice, cover with foil, then bake for 30 mins until the fruit has softened.
Mix the ground almonds, zest and flour with the salt and baking powder in a food processor, then whizz in the butter until it disappears. Pulse in the sugar, tip into a large bowl, then make a well in the middle. Warm the yogurt, milk and vanilla together in the microwave for 1 min or in a pan; it should be hot and may well go a bit lumpy-looking. Tip into the bowl and quickly work into the flour mix using a cutlery knife. As soon as it's all in, stop. The mix should be stiffish but spoonable.
Take the plums from the oven, uncover, then top with 6 big spoonfuls of batter. Scatter with flaked almonds and a little sugar. Bake for 30-35 mins until topping is golden and the fruit bubbles underneath. Scatter with more caster sugar to serve.