Heat oven to 180C/160C fan/gas 4. Gently heat the prunes and Marsala over a low heat until hot. Remove from the heat and leave for about 1 hr or until the prunes have absorbed almost all of the liquid. Whizz the prunes and any remaining booze in a food processor until roughly chopped.
Butter and line the base of a 20cm springform tin. Dust with cocoa, tip out any excess and reserve. Melt the butter and chocolate in a heatproof bowl set over a pan of barely simmering water. Stir in the prune mix.
Whisk the sugars, 2 eggs and 2 egg yolks until pale, thick and doubled in volume. Fold the chocolate mixture and vanilla into the egg mixture, then sift over the ground almonds, flour and remaining cocoa. Fold until everything is combined.
Whisk the egg whites until stiff and gently fold into the mixture. Spoon into the prepared tin and bake for 20-25 mins or until a light crust has formed. Allow to cool in the tin - it will sink and crack as it cools. Remove from the tin and dust with cocoa before serving.