Cut each plum half into 3 wedges and set aside. Combine the sugar, cinnamon and other spices in a bowl and set aside.
Beat the butter, sugar and vanilla until pale and light. Gradually beat in the eggs.
Combine the flour, salt and baking powder and gently mix into the butter mixture. Don’t over beat or the cake will be tough.
Spread half of the batter in the base of the loaf tin (app. 25cm x 13cm and 7½ cm deep) lined with parchment paper, then scatter over half of the plums. Sprinkle half of the brown sugar mixture over the top. Repeat with the remaining batter, plums and sugar.
Bake in the oven at 150°C fan for about 1 h 10 minutes or until a skewer inserted into the centre comes out clean. Cover the top loosely with foil if getting too brown. Cool completely before removing from the tin.
Combine icing sugar with enough lemon juice to make a smooth, thick but pourable icing.
Serve dusted with icing sugar or drizzle with lemon icing.