You may be referring to what in one of my sources is called "chlorine detonating gas" flame, from the combustion of H2 in Cl2.
The temperature of the chlorine detonating gas flame is stated to be about 2,200 C.
Hot, but not as hot as an Atomic Hydrogen (H2 passed through an electric arc, which releases the large required cleavage energy upon recombining) which is put at over 4,000 C!
Now, an idea I had (I let someone else do the work and perhaps mention me) is that since Atomic H2 has a half-life of a few tenths of a second, it is possible to combine the atomic hydrogen in a second region per my source ("Concise Encyclopedia Chemistry" by deGruyter). So what if the 2nd region contained Cl2? If the torch temperatures were additive, we are talking over 6,000 C for an "Atomic H2 Chlorine Detonating Flame"!!!
I have not checked if someone has already invented it, but sounds like a hot idea (pun intended)!