The Shulhan Aruch (Orah Haim 168) establishes that if one eats a significant quantity of "Pat Ha’ba’a Be’kisnin" (the precise definition of which will be discussed in a separate installment), such as cake, then he must wash Netilat Yadayim, recite "Ha’mosi" before eating, and recite "Birkat Ha’mazon" after eating, as though he was eating bread. Specifically, if one eats 216 grams or more of "Pat Ha’ba’a Be’kisnin," then he must treat the product as bread.
If one intended to eat 216 grams of "Pat Ha’ba’a Be’kisnin" but then decided to stop eating before he consumed this amount, then he recites "Al Ha’mihyah" instead of Birkat Ha’mazon. Even though he had washed Netilat Yadayim and recited "Ha’mosi," planning to eat the amount of 216 grams, nevertheless, since in the end he did not eat this amount, he recites "Al Ha’mihya."
Conversely, if one recited "Mezonot" over "Pat Ha’ba’a Be’kisnin," anticipating eating less than 216 grams, but in the end, he indeed ate this amount, then he must recite Birkat Ha’mazon after eating. If, at the time he changed his mind, he planned to eat an additional 216 grams, beyond what he already ate, then he must wash Netilat Yadayim and recite "Ha’mosi," since he is now planning to consume this amount of "Pat Ha’ba’a Be’kisnin." If, however, he decided to eat a quantity that is less than 216 grams, but combined with what he already ate amounts to 216 grams, then although he must recite Birkat Ha’mazon after eating, he does not need to wash Netilat Yadayim or recite "Ha’mosi," since he is not planning now to eat 216 grams.
Summary: If a person plans to eat 216 grams of "Pat Ha’ba’a Be’kisnin" – such as cake – then he must wash Netilat Yadayim and recite "Ha’mosi," just as he would if he were eating bread. And if he indeed ate this amount, then he must recite Birkat Ha’mazon. If he stopped eating before consuming this amount, then he recites "Al Ha’mihya." If he had planned on eating less than 216 grams, but then decided to continue eating such that he will end up eating this amount, then after eating he recites Birkat Ha’mazon. If, at the time he changed his mind, he planned to eat 216 grams more, then he must at that point wash Netilat Yadayim and recite "Ha’mosi."
