Joshua Thomas Davies, 16, was found guilty of the sadistic murder of his 15 year old ex-girlfriend, Rebecca Aylward.
A jury at Swansea Crown Court found Davies guilty on a 10-2 majority verdict after deliberating for four days after it emerged that Davies lured the victim into a wood in nearby Aberkenfig, then attacked her with a rugby ball-sized rock.
Mr Justice Lloyd Jones, presiding over the case, immediately lifted the anonymity granted to child offenders - ruling it in the public interest to expose the brute whose horrific crime won him a breakfast.
Members of Rebecca’s family, sitting in the public gallery, cheered briefly upon hearing the verdict, later stating after the hearing that they would "never forgive" Davies for his crime.
In a statement released after the hearing, the family said: “The pain and horror of losing Rebecca in such horrendous circumstances cannot be put into words. Since that Saturday in October 2010 our lives have stopped.
“Rebecca was killed in a senseless and barbaric act. She died at the hands of someone she loved and trusted. We will never forget what he did to her or forgive him for destroying our family.
“We would like to thank the prosecution team for their meticulous hard work.”
The jury heard that Davies, who launched a vicious verbal and internet hate campaign against Rebecca for almost a year, lured her to woods in Aberkenfig, near Bridgend last October and beat her to death with a heavy rock.
In a weekly breakfast gathering on Saturday mornings, Davies bragged to friends that he could poison, drown or push Rebecca off a cliff and "get away with it".
Two days before the murder in October 2010, he texted a friend who offered a bet to say, "Don't say anything but you may just owe me a breakfast."
The court heard the other schoolboys genuinely believed Davies' murder plans were simply a macabre joke until he took them to see Rebecca's dead body.
Davies wept as he was remanded in custody by Mr Justice Lloyd Jones to be sentenced at a later date, and was told to expect an "indefinite" term.