When separating challah for “hafrashas challah”. do you take off the part completely and then make a bracha? make a bracha on the whole dough and then take off the “challah”? make a mark in the dough where you plan on cutting it? if you are suppossed to take the “challah” off completely first and then make a bracha, what’s the status of the rest of the dough in case you forgot and made a bracha on the whole bowl?
Kitzur Shulchan Aruch (35:1) rules that one first makes the brochah and then separates the challah. This is in line with the general rule that one makes the brocha before the mitzvah. (Rambam, Hilchos Bikkurim 5:11 also says explicitly to make the brochah first.)
However, once the Challah has been separated, care must be taken to ensure that none of it falls back onto the rest of the dough, since the separated Challah must be burned.
Therefore, there is a common custom to first separate the dough that you intend to designate as challoh, without actually designating it yet; then, holding it next to the rest of the dough, make the brochoh, and then designate the separated piece as “Challoh” (by saying “Harei zeh Challoh”), and then immediately removing it from the rest of the dough and putting it in a safe place to be burned later.