Middleton, on loan from Rangers, saw his 75th-minute effort low to his right saved by Hibs goalkeeper Matt Macey while Saints were leading the game 1-0.
However, his miss did not matter as St Johnstone held on to their lead, with Middleton revealing that he had scored all of his efforts in training by going high into the net.
"I don’t know what I was doing,” confessed Middleton. “On Friday I took ten penalties and put them all in the top corner!”
There was relief etched on Middleton’s face, but also joy as he savoured an incredible moment for him and his team-mates.
“I think there was more pressure in the semi-final,” said the 21-year-old. “We put a bit of pressure on ourselves against St Mirren because we were desperate to get this moment, and we knew that we could. The way it has happened in the final is amazing.
"It’s a brilliant group. I’ve loved it from minute one being here. All the lads have welcomed me and the celebrations show how together everyone is.”
Middleton was a goalscoring super-sub against St Mirren in the semis, but was handed a start for the showpiece. "I was delighted to start the game, but either role would’ve been fine – I was just happy to be involved,” he added. “Today is a special moment.”