We have a case of Shabbos that goes into Yom Tov (i.e., Yom Tov commences on motzoi Shabbos).
A lady recited the blessing over lighting Yom Tov candles and prior to lighting remembered that she forgot to say Baruch hamavdil bein kodesh l’kodesh.
What should she do? Should she recite Baruch hamavdil and then light, as perhaps it is not a hefsek since she is forbidden in doing all melacha prior to having recited this phrase?
She is allowed to say Boruch Hamavdil before lighting her candles and it isn’t considered a hefsek since it is speech that is needed in order to be able to light her candles.
O:CH 167-6, M:B 37-40, if something is needed for the meal one may speak out between the bracha and eating, even if it is not related to his actual eating, rather for a halachic restriction, such as feeding the animals. Here too there is a need to say something in order to be able to light the candles and it isn’t a hefsek. R’ Y. Cahen shlit”a