Question: Do I need to sit down for melave malka even when Shabbos goes out after 11pm?
Answer: The Gemara (Shabbos 119b) writes that one should set one’s table for a meal after Shabbos. This meal is known as melave malka, escorting out the Shabbos queen. Rambam (Shabbos 30:5) and the Tur (OC 300:1) explain that we should escort Shabbos out with the same respect that we brought it in with. Thus, the Mishna Berura (300:1; 3) writes that one should lay the table properly as one does for Shabbos, light candles and sing zemiros.
While the Mishna Berura (300:1) writes that melave malka is not as important as the three Shabbos meals, the poskim stress the importance of this meal and the reward of those who are particular to partake.
R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 63:3) quotes many poskim who write that women are equally obligated to eat melave malka (See Maaseh Rav 150).
The Vilna Gaon (Biur Hagra OC 300:1) says that one should eat bread at the meal. The Magen Avraham (OC 300:1) writes that when this is difficult because Shabbos ends so late, one should have a snack instead (See Aruch Hashulchan OC 300:3; Mishna Berura 300:1). At the very least, one should have a hot drink (Shemiras Shabbos Kehilchasa 63:8).
The Baer Heitev (300:1) and Aruch Hashulchan (ibid) quote the Elya Rabba who writes that if one ate shalosh seudos late, and carried on eating after it was dark out, then one has fulfilled one’s obligation and doesn’t need to eat again after Shabbos. Not everyone relies on this leniency, however (see Shemiras Shabbos Kehilchasa 63:6).