Preheat the oven to 350°F (170°C).
Grease the bottom and sides of an 8 inch loose bottomed cake pan.
Using an electric mixer, cream the butter and sugar together.
Beat the eggs and vanilla into the creamed mixture.
Sift together the flour, baking powder, cinnamon, ginger, nutmeg and salt.
Add the flour mixture to the batter and mix just until blended.
Pour in the buttermilk and beat until smooth.
Pour the batter into the prepared pan. The batter will be thick and you will have to spread it to fill the pan, coaxing it out to the edges. Smooth the top of the batter so it is even.
Arrange the plum slices over top of the batter in a circular fashion, leaving a bit of space between each slice.
Sprinkle the Demerera sugar over the top.
Bake for 35 to 40 minutes or until a skewer inserted in the centre of the cake comes out clean.
Allow to cool in the pan on a wire rack for at least 30 minutes.
When the pan is cool enough to handle, release the springs of the cake pan and gently remove the sides. The bottom of the cake pan can be gently removed by sliding a knife underneath the cake and lifting the cake on to a wire rack using a large slice. However I often cheat and leave the cake on the metal base for serving to avoid any risk of the cake not coming off in one piece.