Preheat the oven to 180°C.
Add the oil and butter to a small pan and heat gently for about a minute until the butter has melted, then remove from the heat.
Meanwhile prepare a 22cm x 22cm tray bake tin by greasing with a little butter and lining with baking parchment.
Sieve the self-raising flour into a large bowl and mix in the oil and melted butter mixture, caster sugar, eggs, cinnamon, nutmeg, allspice and vanilla extract all together until a smooth batter is formed.
Place the prune halves in 3 rows on top of the sponge (you’ll have an odd half left).
Place on the middle shelf of oven and bake for 35 minutes or until golden on the top and a skewer comes out of the centre of the tray bake clean.
Remove from the oven and leave to cool on a wire rack.
Once the cake has cooled, mix together the sieved icing sugar and water and drizzle over the cake.
Leave to set and slice into squares to serve.