Heat oven to 180C/fan 160C/gas 4. Butter and line a 20 x 30cm baking tin with baking paper. Put the butter, sugar and ground almonds into a food processor, then pulse until the mixture resembles very rough breadcrumbs. Spoon out half the mix into a bowl and set aside.
Add 140g flour into the mix in the processor and whizz until it just forms a dough. Tip into the tin and press down with the back of a spoon. Bake for 15-20 mins until golden. Leave to cool for 10 mins.
To make the filling, put the remaining butter and the sugar and almond mix back into the processor, saving a few tbsp for the topping. Add the eggs, the 25g flour, cinnamon and baking powder and whizz to a soft batter. Spread over the base.
Top with the plum pieces and a little extra caster sugar and cinnamon. Bake for 20 mins, then sprinkle with the remaining crumble mix and flaked almonds. Cook for another 20 mins or until golden. Leave to cool in the tin before slicing.