#825: “Oh, no! I forgot to say the Brachah before separating Challah!”
The procedure of hafrashas challah (separating a portion of bread dough)* is to make the brachah, separate a piece of dough and to declare aloud the separated portion as challah: “Harei zu challah” (this is challah). Poskim mention that verbally confirming hafrashas challah is an offshoot of the practice of krias shem (calling by name, i.e., verbal identification) when separating terumah (gifts [for the kohen in the Land of Israel])—“harei zu terumah.”
(There is a machlokes haposkim—halachic dispute—regarding when to say the brachah on hafrashas challah: do we recite it before separating the challah or before krias shem? The consensus is that we should recite the brachah before separating the challah from the dough.)
If a person forgot to make the brachah before separating the challah, can they still recite it afterwards? The answer is: If their intention was to then declare the dough “challah” (or this is the individual’s practice on a regular basis), they may still say the brachah post-separation. Since they will proceed to make a verbal statement concerning the separation, the hafrashah has not yet been actualized. However, for those who do not plan to perform a krias shem the mitzvah was completed with the physical separation of dough, and a brachah post-facto constitutes a brachah levatalah (a brachah in vain).
*The shiur ([minimum] amount) of dough required to separate challah is a volume of flour greater than forty-three beitzim (eggs). For other conditions for this mitzvah, see Halachah #231; Halachah #318; Halachah #503; and Halachah #588.