PLUM VANILLA AND SOURED CREAM CAKE

150g unsalted butter, softened, plus extra for greasing • 400g Waitrose & Partners British Plums (about 4-5 plums) • 200g golden caster sugar • 3 medium British Blacktail Free Range Eggs • 150g plain flour • 100g ground almonds • 2 tsp baking powder • 1⁄4 tsp fine salt • 150g soured cream, plus extra to serve • 11⁄4 tsp vanilla bean paste • 2 tbsp plum jam