Preheat the oven to 350°F (170°C).
Cut the plums in half and remove the pits.
Grease an 8 inch springform or loose bottomed cake tin and line the bottom with baking parchment.
Cream the butter and sugar together in an electric mixer.
Beat in the eggs and vanilla.
Sift together the flour, baking powder, cinnamon, ginger, nutmeg and salt.
Fold the flour into the batter.
Beat in the buttermilk.
Pour the batter into the prepared pan, spreading it to fill the pan and smoothing the top a little.
Arrange the plum halves, skin side up, in a circular pattern on top of the cake.
Sprinkle the top of the cake and the plums with the 2 tablespoons of sugar.
Bake for 35 to 40 minutes or until a skewer inserted in the centre of the cake comes out clean.
Cool in the pan for 20 to 30 minutes.
When the pan is cool enough, remove the cake and cool on a wire rack.