A steamer or half fill saucepan with water and bring to a boil. Grease four individual pudding molds and sprinkle each with a little brown sugar to coat well all over.
Peel and core apple. Dice into a bowl, add the raisins and mix well. Divide equally among the pudding bowls.
Place remaining brown sugar into a mixing bowl. Add butter, eggs, flour, baking powder and cocoa; beat until combined and smooth.
Spoon mix into the molds and cover each with a double thickness of aluminum foil. Steam for 45 mins, adding boiling water as needed to pan, until puddings are well risen and firm.
For hard sauce, cream the butter and icing sugar till smooth. Add rum/whiskey to taste, in a slow stream while beating steadily to incorporate. Stir before serving over the pudding or other fruitcakes etc. (This sauce can be refrigerated for weeks and just needs to come to room temperature.)
Run a knife around the edges of each pudding to loosen it, and turn out onto individual plates.
Serve immediately with the hard sauce.