"After all this time, there's no way I'm letting [you] go that easily."
The card with Henry's serious picture holding a tea cup in the mystery section, once having uncovered his mystery, turns over to reveal a smiling Henry holding the toy robot Randall had given him when they started to be friends.
The credits, full stop. Especially when Little Randall gives a flower to Little Angela, and Randall and his mother are finally reunited, with Dalston running up to him in happy tears.
Randall and Henry as children, particularly when Randall intervenes to stop a maid from picking on Henry. The brotherly affection between them is deeply touching and beautiful to watch.
And it was this kindness in turn that inspired Henry to put everything into finding Randall after he disappeared and, when that failed, to create Monte D'or for the purpose of not just providing a new home where Randall can find him and Angela, (as Stansbury had fallen into ruin and became a ghost town in the 18 year time difference,) but also to protect all that Randall held dear.
As in The Last Specter/Spectre's Flute where Luke suggests they hold hands so as not to get lost in the mist, Miracle Mask has Luke asking Layton and Emmy not to let go of his hand, lest he gets lost in the throng of tourists in the arcade. Considering how shy and withdrawn he was in The Last Specter/Spectre's Flute, it's an adorable hint of the trust he has in the Professor and Emmy.
The picture you get as part of your reward for clearing all 150 puzzles in the game shows Layton's group of old friends (Oh, and Luke too.) all grown up, and all smiling. Randall is even trying to take Layton's hat off. Not only is it funny, but seeing them together again and finally smiling and laughing, especially considering all the crap they've been through, is incredibly cute and heartwarming.