http://trialanderrorhomeec.blogspot.com/2011/12/ktt-using-sour-milk.html
The relevant portion I want to remember I'm posting myself, just to make sure it doesn't get lost if the link does.
From "The Royal Guide to Meal Planning:"
Thick sour milk, sour cream and buttermilk...can each be used in place of sweet milk if these instructions are carefully followed. Use one-third teaspoon soda to each cup of thick sour milk and add [baking powder] as directed in the recipe for sweet milk.And another link with recipes that I've noticed has been quoted extensively on the net is http://www.cooks.com/rec/story/121/
Because sour cream contains so much fat, the butter or other shortening in the recipe will have to be reduced when sour cream is used.
Soda can be sifted with the dry ingredients or added to the sour milk. Either method is satisfactory with perhaps a little advantage in the former.
Where other acid-reacting materials such as molasses, raisins, bran and brown sugar are also included in a sour milk recipe, the amount of soda will, of course, have to be increased slightly.
And having joined a Mennonite congregation this year, I was fascinated to come across this blog, for which I'm linking to one of the buttermilk recipes I wish to try. Hopefully, I can find my Roger's Golden syrup!
Syrup Platz recipe