This is the Bahia Tyrannulet, an Endangered species, discovered in 1992, and known only from a few localities since then. Serra Bonita is one of them. (Interestingly when some ornithologists visited Serra Bonita to look for this species, they ended up finding another species new to science - the strange Pink-legged Graveteiro - but that's another story).
Ciro and I saw a Bahia Tryannulet high in the canopy and I managed to get a shot in which the bird was reasonably sharp but had to be cropped to 100% to see anything. Ciro thought it was the best shot yet of the species and I was thrilled!
Next day though, we got another chance with a close bird that was moving with a mxed flock and was eye-level for us below the ridge. What a thrill! Serra Bonita, Bahia, Brazil, Dec 2011.