If one’s meal [i.e. Seudas Moshiach] on the last day of Pesach continues past nightfall, may one eat Chametz during this meal?
One whose meal on the last day of Pesach continued into Motzei Yom Tov, until after Tzeis Hakochavim, is now permitted to eat Chametz during his meal [prior to Birchas Hamazon] even though he did not yet Daven Maariv or recite Havdala at all [and did not even say Baruch Hamavdil Bein Kodesh Lechol]. [In such a case, one is nevertheless to recite Yaaleh Veyavo and Chag Hamatzos Hazeh within his Birchas Hamazon.]
Sparks of Chassidus:
As Pesach reaches a close our souls gain the ability to refine even course matters such as Gebrochts and Chametz. It is for this reason that on the last day of Pesach in the Diaspora one eats Gebrochts, as the soul now has the ability, and obligation, to refine this food, even though it has a suspicion of Chametz. When Motzei Yom Tov has arrived one has ability to now refine even actual Chametz, and nevertheless still recite in Birchas Hamazon Chag Hamatzos Hazeh.”
 Admur 491/3; M”A 491/1 [Begins with Tzaruch Iyun, although gives reason to conclude that it is Mutar]; Chok Yaakov 491/1 [concludes like suggestion on M”A ibid]; Elya Raba 491/1; Chok Yosef 491/1; Shulchan Gavoa 491/1; Poskim in Kaf Hachaim 491/7; See Likkutei Sichos 22/36 footnote 62-64
Other opinions: Some Poskim rule one may not eat Chametz prior to reciting Havdala. [P”M 491 A”A 1; See Kaf Hachaim ibid] Other Poskim rule that while one may rely on the Poskim ibid to eat Chametz prior to Havdala, one may not eat Chametz prior to reciting Birchas Hamazon, being that this will create a contradiction to his recital of Yaaleh Veyavo in Birchas Hamazon, as how can he say Chag Hamatzos Hazeh if he ate Chametz. [M”B 491/1; See Kaf Hachaim 491/8 and Likkutei Sichos ibid who negates this understanding, as it is forbidden to eat after Birchas Hamazon, prior to Havdala, and hence the only case in which the Poskim ibid can possibly allow eating Chametz is if one did not yet say Havdala!]
 Pashut, as explained in previous footnote; Kaf Hachaim 491/8 that so is implied from Setimas Haposkim; Likkutei Sichos ibid; See other opinions in previous footnote and the reason for their negation!
 So is implied from the wording of Admur “and did not yet say Havdala at all”
 The reason: As the prohibition against eating Chametz on Pesach is not dependent on Havdala at all, and since Tzeis Hakochavim has arrived, it is considered night for all matters, and the holiness of Yom Tov has already dissipated. The fact that one is prohibited from doing Melacha prior to Havdala is not due to the holiness of the Yom Tov, but rather due to a Rabbincial prohibition due to reasons explained in 299/15. (Now, although there is a Mitzvah to add onto the Holiness of Yom Tov, this is only with regards to not doing Melacha during the additional time of Yom Tov, as this Mitzvah is learned from the verse “Tishbisu Shabatchem”, as explained in 261/4.) However, regarding other matters which are dependent on the holiness of the day, such as the Mitzvah of Mikra Kodesh, see chapter 529, there is no need to add from the weekday to the Yom Tov. Accordingly, certainly the eating of Chametz, which is not dependent at all on the holiness of the day, as even during Chol Hamoed one is prohibited from eating Chametz, [is permitted to be eaten during the period of Tosefes Yom Tov]. [Admur ibid]
 Implication of M”B ibid; Kaf Hachaim 491/8 in name of Poskim; Likkutei Sichos ibid
 Likkutei Sichos ibid