If one eats a lot of pasta noodles and makes a mezonos, and then decides to eat 2 pieces of cake to finish off the snack, does one make another berachah? Also, does the cake join in with the mezonos and now make one have to bentch if one is full?
Because the cake is more “important” than the noodles (it has tzuras pas, and the noodles don’t), one would have to make a new berachah on the cake — unless one had specific intention upon making the original berachah to continue to eat cake.
Concerning bentching, if before eating the cake one was still hungry, and the cake makes one full, one would have to bentch after eating the cake. In the case described in the question, one would usually be full after eating the noodles, and the cake is being eaten as an additional “appertif.” In this case, one need only recite al hamichyah. Because the cake is being eaten as a “dessert,” and is not the main part of the meal, most poskim maintain that it would not join with the noodles in obligating the full birkas hamazon.
Sources: Re: first berachah — see Mishnah Berurah 211:32; re: after berachah — see Vezos Haberachah, p. 33-34, quoting from various authorities.