You may recall that this specific activity was first on a list of specific tortures in the judgment of the International Military Tribunal for the Far East (AKA the Tokyo War Crimes Trial). These are one part of count 54 in the indictment:
"Ordered, authorised, and permitted inhumane treatment of Prisoners of War (POWs) and others."
This was one of several capital charges in the indictment, and quite a few Japanese officers and politicians, including generals, admirals, and the Prime Minister, were convicted of this charge and executed.
In the US, the Uniform Code of Military Justice covers the prosecution of members of the armed services for crimes like these; this would cover the officers and enlisted personnel who were involved, but the President himself, while in the military chain of command, does not appear to be subject to the UCMJ. In order to try Mr. Bush for a war crime, it appears that the US would either have to sign on with the International Criminal Court or establish a special military tribunal just for him.
When considered as a Federal criminal offense, the act Mr. Bush confesses to is covered by 18 U.S.C. Sec. 2340, which addresses torture under color of law. This would be prosecuted as an ordinary crime in Federal court, good for 20 years per count (if the victim survived) or death (if the victim died).
It appears that the US Attorney's Office for the District of Columbia would be responsible for the prosecution. Given the public confession and the ample numbers of surviving witnesses in Guantanamo Bay and elsewhere, the trial should be straightforward. The FBI has a web page discussing how to report this class of crime.