Preheat the oven to 180C/350F/gas mark 4. Grease and line a 30x23cm/12x9in cake tin.
Place the sugar, eggs, oil and vanilla in a large bowl and whisk with an electric mixer for 5 minutes until pale and fluffy. Add the zest and mix again.
Sift together the flour and baking powder and gently fold it in using a metal spoon until it is fully combined. Add the poppy seeds and mix again.
Pour the batter into the lined tray and spread it level. Arrange the plum halves on top, cut side down. Bake for about 40-50 minutes, until the cake is firm and a skewer inserted into the centre comes out clean.
Leave to cool in the tin. Before serving, mix together the icing sugar and cinnamon and sift it over the cake.