For the Chocolate Malteser Cake which type of Horlicks should I use - original or chocolate flavored?
Nigella's Chocolate Malteser Cake (from Feast) is given a Malteser (malted milk ball) flavour with the addition of malted milk powder. There are two well-known brand names, Horlicks and Ovaltine, but a store own brand malted milk powder would also be fine. You should be using the traditional, just malt, version and not an alternative flavoured version.
We would also mention that you need to use the type of malted milk powder that is added to hot milk, and not the "instant" type that would be added to boiling water. The instant drinks contain skimmed milk powder and could affect the finished cake.