Only if it was a k’zayis and if you had in mind that it was Shushan Purim and that you were eating it as a segula to be zoche to keep Shushan Purim next year in Yerushalayim Habenuya, and if you included that in the l’sheim yichud.
The best thing to have done would have been to have eaten a second k’zayis of hamantashen after tzeis hakochavim and to have had in mind that if the first one was eaten b’zman, then the first one should count for the first day of Purim and the second one for Shushan Purim, but if it was really already Shushan Purim, then the first one should count for Shushan Purim and the second one should be tashlumim for the first day.