There is a requirement to recite one hundred Berachot each day. On Shabbat, this requirement poses somewhat of a challenge, due to the fact that the Amida consists of just seven Berachot, as opposed to the weekday Amida which consists of nineteen Berachot. For this reason, many people have the practice of eating snacks and smelling Besamim at various points over the course of Shabbat in order to reach one hundred Berachot.
One strategy that some people employ is to recite Birkat Ha’mazon before dessert at the Shabbat meals. By reciting Birkat Ha’mazon before dessert, one gains another Beracha, or perhaps two Berachot, as he will then have to recite a Beracha Aharona (Me’en Shalosh, Boreh Nefashot) after dessert. (The Beracha before eating would be required in any event, as dessert is not covered by the Beracha of "Ha’mosi" recited over the bread at the beginning of the meal.) This practice, however, is subject to considerable controversy and debate. Some Halachic authorities discourage this practice, as it is forbidden to knowingly put oneself in a position that necessitates reciting Berachot. Others, however, maintain that this is permissible, since the objective is to fulfill the Halachic requirement of reciting one hundred Berachot each day.
Hacham Ovadia Yosef addresses this topic in Yehave Da’at (vol. 6), and he sides with the opinion of the Shela, Elya Rabba and Rav Zalman of Liadi, who permit reciting Birkat Ha’mazon before dessert for the sake of reaching one hundred Berachot. However, Hacham Ovadia adds that it is proper in such a case not to bring the dessert to the table until after Birkat Ha’mazon.
Many people have the custom of smelling Besamim before Kiddush on Friday night to add another Beracha. This custom is mentioned by the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), who adds that he had the practice of smelling Besamim a second time after the meal. Hacham Ovadia Yosef notes that this is permissible only if one had entirely forgotten about the Besamim during the meal and then recited Birkat Ha’mazon. Otherwise, a new Beracha over Besamim should not be recited after the meal.
Summary: It is customary to eat snacks and smell Besamim on several occasions over the course of Shabbat in order to ensure to fulfill the Halachic requirement to recite one hundred Berachot. It is permissible for this purpose to recite Birkat Ha’mazon before dessert so that additional Berachot will be required after dessert, though the dessert should not be placed on the table until after Birkat Ha’mazon.
