I have tried some of your cakes and muffin recipes and they have turned out well. Ooi H.K., Subang Jaya Buttermilk is a cultured milk product. It is made from pasteurised skimmed milk and is mildly acidic with a creamy taste. You can substitute buttermilk by adding a tablespoon of lemon juice to 250ml UHT milk. Stir well to mix then set aside for 10 minutes before using.
Sour cream is a single cream that has been soured commercially.
It has a higher fat content than yoghurt. If it is difficult to obtain, you can add one tablespoon lime juice to about 35 to 40ml fresh single cream. Set aside for half an hour before use. So, tepid water is lukewarm water. What you should do is mix hot water with cold water until it is lukewarm to the touch when tested on the palm of your hand.