The search for evidences of the formation mechanism of SMBHs requires us to access, with X-ray observations, the Universe at z>5. This is a challenge because the deepest Chandra observations combined with HST-WFC3 provide only a handfull of candidates high-z AGNs to use to test SMBH formation machanisms. Other than that we need to search for their footprints in cosmic backgrounds. Together with a novel measure of the CXB spectrum and of its unresolved component, I am presenting recent observational result that may shed light on the first black holes in the Universe. In particular, the recently detected joint fluctuations of the Chandra CXB and and the Spitzer CIB are opening a new window on this epoch of the cosmic history and paving the way to JWST, WFIRST, Euclid, The X-ray Surveyor and Athena.