「明日のパーティー、来る？」「もちのろん！」
"Are you coming to the party tomorrow?" "Of course!"
It goes without saying that he'll help out.
Kanji spelling is a jocular ateji rendering; rarely used in practice.
A playful elongation of 勿論 (もちろん), splitting the word into もち and ろん and inserting の for humorous effect.