Juventus had performed an unbelievable comeback to tie the aggregate score at 3-3 before Lucas Vazquez was fouled in the penalty area in the dying seconds.
Buffon was sent off in the aftermath for raging at referee Michael Oliver but Ronaldo was not impressed with the remonstrations.
The Real Madrid forward duly stepped up to smash the resulting spot kick into the top corner past substitute shot-stopper Wojciech Szczesny.
The goal sparked wild jubilation from the home supporters as Ronaldo wheeled away having rescued his side’s European hopes once again.
But after the match he was less than impressed with Buffon for getting sent off.
“I don’t understand why they are protesting,” he said.
“Lucas was tackled from behind and if they don’t give him the penalty, it’s a goal.”
Meanwhile, Buffon was furious with Oliver and was captured in the tunnel venting his anger further.
“The referee has a trash can for a heart,” the goalkeeper said.
Ronaldo also explained how his heart was racing as he prepared to take the spot kick.
But the 33-year-old revealed he was able to regain his composure before sending his side through to the semi-finals.
“The pulse increased, but I calmed down, and I knew I’d be decisive,” he added.
“We have suffered, but it helps us to learn.
“In football, nothing is given away, you have to fight until the end.
“Madrid could’ve scored more goals, but Juve and Buffon were good.”
Juventus manager Massimiliano Allegri was equally as disgusted with the result following the late heart-break.
And the Italian explained how he had already begun planning for extra-time.
“I have a bitter taste as we didn’t have the chance to go to extra-time,” Allegri said.
“Both sides would’ve deserved that I think.
“Small details changed the whole tie unfortunately.
“I had two more substitutions planned for extra-time and I was confident we could do it.”
Real join Liverpool, Bayern Munich and Roma in Friday’s semi-final draw.
