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Heat oven to 180C/160C fan/gas 4. Grease and line a 22cm deep, round cake tin (preferably with a loose bottom) with baking parchment.
First make the topping: put all the crumble ingredients in a bowl and rub together until crumbly.
For the cake, put the butter, sugar, eggs, flour, baking powder, vanilla, ground almonds and soured cream in a big bowl. Beat with an electric whisk until smooth, then tip into the tin and level the top. Arrange the plums, cut-side up, over the mix, then scatter over the crumble.
Bake for 50 mins-1 hr on a middle shelf until a skewer poked into the cake mixture comes out clean (plums will remain sticky, however). Cool in the tin for 20 mins, then finish cooling on a wire rack. Serve plain or with soured cream or yogurt. Keeps for 3-4 days in an airtight cake tin.