What a beautiful custom it is when we return from Shul on Friday night and bless our children. Could there possibly be a halachic issue with it? The answer is yes…….
The Gemara in Ketubot (24b) states that a “Yisrael” (i.e. non-Kohen) that blesses Birkat Kohanim transgresses a prohibition. Rashi states that the Mitzvah was given to the Kohanim alone and can not be done by a regular Jew.
Tosefot ask a question from another Gemara which states (Shabbat 118b) the confession of Rabbi Yosi that despite the fact that he was not a Kohen, he used to duchen when asked to do so.
The Sefer Haeshkol answers that the prohibition is only one of a “bracha levatala” (a blessing in vain) and when Rabbi Yosi used to duchen, he did not utter a bracha and therefore did not transgress any prohibition.
The Maharsha answers in a different way: the prohibition is only when one blesses in the way that Kohanim do – i.e. with 2 hands. A one-handed blessing would however not be problematic. Hence the problem is only in the way the Blessing is made and not in the content of the blessing.
The Ktav Sofer has an additional answer: the prohibition is only in force when one has the intention to fulfill a Mitzva as prescribed by the Torah. When one merely intends on blessing a person out of devotion and not as a Divine decree, then this is not only permitted but even commendable. (This answer also sheds light on a very difficult Halacha which requires a person who finds Tefillin in the middle of a street on Shabbat to put them on and walk home. Behold one is not permitted to wear Tefillin on Shabbat and is in fact liable for transgressing the prohibition of “bal tosif” (adding to the Torah)).
There are many authorities that follow the ruling of the Maharsha and therefore require a person to bless his children on Friday night whilst only placing one hand on his head.