Perfect with a cup of tea or coffee or a hot dessert with cream, custard or ice-cream.
Preheat oven to 180C/160C Fan /Gas 4.
Grease a 23cm/9inch springform cake tin and line the bottom with greaseproof paper.
Mix the flour, cinnamon and salt together.
Rub the butter into the flour mixture with your fingers to start forming a crumble.
Stir in the hazelnuts and brown sugar.
Sprinkle over 1 tbsp water and bring the mixture into different sized pieces of crumble.
Sift the flour, cinnamon and salt together then set aside.
Beat the butter and caster sugar together with electric beaters.
Now beat in the egg and vanilla until light and fluffy.
Beat in half the flour mixture, then half the milk until well combined. Then add the other half of flour and the rest of the milk. Beat until smooth.
Transfer the cake batter to the prepared tin and smooth the top.
Arrange the quartered plums over the top of the batter, pressing down slightly.
Sprinkle the crumble mix over the top of the plums and put the cake in the preheated oven.
Bake until risen and a skewer comes out clean (65-70 min). Check after 20 min and if the cake is browning too quickly, cover with a sheet of foil.
When cooked, place on a wire rack for a few minutes, then run a knife around the edge to prevent the fruit juices sticking to the tin. Remove the outer tin and leave the cake on its base until completely cold before removing the base and greaseproof paper.