If one eats 216 grams or more of "Pat Ha’ba’a Be’kisnin" (the precise definition of which will be discussed in a separate context), such as an entire box of cookies, then he must treat the cookies as bread with respect to Halacha. He thus washes Netilat Yadayim, recites "Ha’mosi" before eating, and recites Birkat Ha’mazon after eating. And, he does not recite Berachot over other foods or beverages he eats with the cookies, just as one does not recite Berachot over foods or beverages in a meal once he recited "Ha’mosi" over bread.
Normally, one recites a Beracha Aharona after eating only if he ate a "Ke’zayit" within the time-frame of "Kedeh Achilat Paress," which, according to Hacham Ovadia Yosef, is a period of 7.5 minutes. In the case of one who eats 216 grams of "Pat Ha’ba’a Be’kisnin," Hacham Ovadia ruled that he treats the product as bread if he eats each "Ke’zayit" within 7.5 minutes, and that there is no break in his eating of 7.5 minutes or more in between the consumption of one "Ke’zayit" and the other. For perspective, 216 grams of cake or cookies consist of approximately 7.5 "Ke’zetim." As long as each "Ke’zayit" is eaten within 7.5 minutes, and there is no 7.5-minute break between one "Ke’zayit" and another, one must wash Netilat Yadayim, recite "Ha’mosi" before eating, and recite Birkat Ha’mazon after eating.
Summary: If one eats 216 grams of cookies or cake, then he must treat the cookies or cake like bread with respect to Netilat Yadayim, the recitation of "Ha’mosi," the recitation of Birkat Ha’mazon, and an exemption from reciting Berachot over other foods eaten during the meal. This applies only if the person eats each "Ke’zayit" of cake or cookies within a period of 7.5 minutes, and there is no break in eating of 7.5 minutes or more.