The Shulhan Aruch (Orah Haim 168) establishes that if one is reciting a Beracha over bread, and he has before him a whole loaf of bread and a slice, he should recite the Beracha over the whole loaf. A whole food product is considered more respectable than a piece that had broken off, and therefore, out of respect for the Beracha, one should preferably recite the Beracha over the whole loaf and then eat a piece from the loaf. Of course, his Beracha also covers the slice of bread.
The Shulhan Aruch in this context addresses the common case of loaves of bread which were attached when they came out of the oven, and were then separated. He writes that if the separation was done cleanly, such that each loaf appears whole, then each is considered "whole" with respect to Berachot, and it takes precedence over a slice. When one recites the Beracha over such a loaf, he should preferably cut from the side that was not attached to another loaf.
In general, it is preferable to cut a loaf in the portion that is fullest baked.
The preference for whole products when reciting a Beracha applies also when reciting "Mezonot." Therefore, when one recites a Beracha over cake, for example, it is preferable not to break it into pieces until after reciting the Beracha. And, if one is planning to eat both a piece of cake and a cookie, he should recite the Beracha over the cookie, since it is a whole product, and then eat a piece of the cookie. Of course, this Beracha also covers the piece of cake.
Summary: If one is reciting "Ha’mosi" over bread, and has in front of him a whole loaf and a slice, he should preferably recite the Beracha over the whole loaf and then eat a piece of the loaf, before eating the slice. A loaf that was attached to another loaf in the oven and then separated is considered a whole loaf in this regard, as long as the loaves were separated cleanly. When reciting "Mezonot," too, it is preferable to recite the Beracha over a whole food item, and therefore, one should not break his cake, cracker or cookie until after reciting the Beracha. If one is eating a cookie and a piece of cake, he should preferably recite the Beracha over the cookie.