Cut the cake into slices and arrange on a plate or wide, shallow dish. Drizzle with the orange liqueur.
Zest the orange or clementines into a bowl and leave the zest to one side. Then squeeze the juice from the orange or clementines, pouring this over the liqueur-soaked cake.
Whip the cream until thick but softly so, and spoon unfancily over the top of the saturated, not to say gloriously sodden, cake.
Arrange the blackberries over the top of the whipped cream, then scatter with the reserved zest.
Recipe from Nigella: Kitchen by Nigella Lawson (published by Chatto & Windus).