Heat the oven to 180C/160C fan/gas 4. Oil and line the base of a deep 20cm springform cake tin. Combine the butter with 50g of the light brown soft sugar and spread onto the base of the lined tin. Arrange the plum wedges on top in concentric circles.

Tip the buttermilk, vanilla, oil, remaining sugar and eggs into a large bowl and beat for 1-2 mins using a fork or whisk to break up the eggs. Tip in the flour and bicarbonate of soda and mix until you have a smooth batter. Carefully pour this on top of the plums, then put in the oven for 45 mins to 1 hr until a skewer comes out clean. Leave to cool in the tin completely, then remove from the tin and invert onto a board or serving plate.