Both are correct. Usually in speech, we use the plural “curds”. Remember the nursery rhyme “Little Miss Muffet”? She was sitting on her tuffet and eating her “curds and whey”. We ask someone whether he would like to have “curds”. When the word is used before a noun, it has to be “curd” and not “curds”. We ask someone if he would like to have “curd rice” or “curd cheese”. In both these cases, the word “curds” cannot be used. Americans do not use “curd”, they prefer to use the word “yoghurt” instead.