Purple plums aren’t usually sold by variety in France, but tart ones work best, such as Santa Rosa plums. You could use other kinds of plums, such as Italian prune plums (quetsches), although I prefer to use the tartest ones I can find. If you wanted to experiment with other types of fruit, this cake probably lends itself to others.
The toffee is drizzled over the top in a modest quantity. But if you are a toffee-lover, you can certainly double the amount and really go for it. Buttermilk is called lait ribot or
lait fermentè in France and is available in some supermarkets and Arab markets. You can make a good substitute by using a scant 1/2 cup (125ml) of whole milk with a teaspoon of lemon juice or white vinegar added. Let stand for 10 minutes, then use the soured milk just like buttermilk.