"It was fantastic, a great moment for myself and we deserved it," Beattie told Sky Sports News. "We came with a gameplan to frustrate Celtic and I think we did that.
Rudi Skacel had given the underdogs the lead, only for Gary Hooper to score what seemed to be the equaliser three minutes from time.
"To come back after they scored to equalise, I'm delighted," Beattie added.
"I was thinking [of Celtic keeper Fraser Forster] please dive out of the way quick enough when I stepped up to take it. I don't normally take penalties but I've practised a couple during the week."
Hearts will meet Edinburgh rivals Hibernian in the final for the first time since 1896 on May 19.