The 19-year-old right-back made his only his second top-team appearance for the Gorgie outfit as they defeated Inverness on penalties in Sunday's Easter Road semi-final to book a Hampden date against St Mirren on March 17.
The Glasgow-born defender was one of seven players aged 21 or under in John McGlynn's starting line-up. Having shown faith in his young players for the big occasion once, Mullen now hopes the Tynecastle boss is of the same mindset in six weeks' time.
He told Press Association Sport: "It was great to get the chance to play in the semi and to play in the final would be even bigger. Now I just need to wait and see if the manager will play me again.
"I was a wee bit nervous before the game but once I got out on to the pitch, some of the more experienced players told me just to treat it like any other normal game, not to let the crowd get to me and that I'd be fine. They were right.
"It helped having the other young lads in the team. We have all played together since we were young kids and we are used to being out there together.
"So when you have your mates beside you it does help. You don't feel like it is all coming down on you as one player.
"It was great when we won the shoot-out. We were jumping about with the fans and everyone was going wild. With it being at Easter Road too, it was even more special for the club.
"Three sides of the ground was full of our supporters, so it was almost like a home game for us."