Preheat the oven to 180 C / Gas 4. Grease a 18cm springform cake tin and line with baking parchment.
In a bowl, combine the plums and lemon juice and zest and toss together to coat the plums. Set aside.
In a mixing bowl, whisk the egg whites until stiff peaks form then gradually add in the sugar and egg yolks and continue to whisk until thick and creamy. Sift in the flour and cinnamon and gently fold in using a large metal spoon to retain as much air as possible.
Spoon half of the cake mixture into the prepared cake tin and press in half of the plums, add the remaining mixture and then arrange the remaining plums over the top.
Bake in the preheated oven for 45 to 50 minutes until golden brown, risen and firm, or until a skewer comes out clean when inserted into the centre. Remove from the oven and allow to cool in the tin. Release the cake from the tin and transfer to a serving plate. Just prior to serving, brush the warmed jam over the top of the cake then slice and serve.
Perfect with a cup of coffee or as a dessert for after a meal, served with a little crème fraiche mixed with a little vanilla extract.