Preheat the oven to 180C (350F). Grease a round springform (23cm/9in) cake tin and line the base with baking paper.
Halve the plums, remove the stone and slice each half into 3 segments. Toss them with the four tablespoons of sugar.
Arrange the plums on the bottom of the prepared cake tin in a single layer.
Put the softened butter, sugar, flour, ground almonds, soured cream, eggs, baking powder, salt, spices and zest in the bowl of your stand mixer and beat together until you have a smooth, quite thick batter.
Carefully spread the batter over the fruit and tap the cake tin against the counter.
Bake for about an hour or until the cake is risen, golden and a skewer inserted in the centre comes out clean. Leave the cake to cool in the tin.
Run a knife around the edge of the tin to loosen the cake and release the springform sides. Very carefully invert the cake onto a platter or cake stand.
Warm the jam in a saucepan or the microwave until it is runny. Brush the jam over the top of the cake.
Serve slightly warm with some cream or ice cream. Cake keeps well for 2-3 days.