Preheat oven to 180°C and grease and line a slice tin.
Using an electric mixer beat the butter, sugar and vanilla until pale and creamy, then add the eggs one at a time. Lightly fold through the almonds and flour.
Spoon the batter into the prepared tin, then top with the plums, placing them skin side down. Sprinkle the Demerara sugar and bake in the oven for 35minutes, until golden brown.
Allow to cool before sling into rectangles for the lunchbox.
