1. Preheat the oven to 180 C, gas mark 4. Butter and base line a 20cm loose-bottomed cake tin. Sift together the flour and baking powder. Cream the butter with the caster sugar until pale and fluffy then beat in the egg. Fold in the sifted ingredients and add enough milk to give a soft dropping consistency. Spoon into the tin and level the surface. Arrange the sliced plums over the surface.
2. Sift the flour and cinnamon for the topping together and stir in the almonds and sugar. Pour over the butter and stir with a knife to mix thoroughly. Sprinkle over the plums.
3. Bake for 45 minutes until golden and a skewer comes out clean when inserted in the centre of the cake. Cool in the tin for 5 minutes. Serve warm with clotted cream or enjoy cold.