Preheat the oven to 160ºC/325ºF.
Grease and line with parchment a small cake tin with a diameter of about 8cm by 5cm.
Melt 25g of the butter in a saucepan over a low heat, add the brown sugar and cook until slightly caramelised. Add the halved plums, orange zest and some spice and continue to cook for 2 minutes until lightly caramelised and softened.
Remove from the heat and leave to cool for 5 minutes. Arrange the plums in the base of the prepared tin then pour the cooking liquid.
In a bowl, beat together the remaining butter and caster sugar until creamy and light. Beat in the egg. Sieve in then, the flour and baking powder and combine thoroughly. Spread the mixture over the plums and bake for 30 minutes until the sponge is golden.
Remove from the oven and leave at room temperature to settle for 5 minutes.
Remove the parchment paper on the edges and place the serving plate over the tin.
Quickly turn the cake upside-down and gently lift the cake tin.
Peel the parchment paper and serve warm with whipped cream.