I know Moroco (Bay Station) gets a lot of criticism on this board, but they do have an excellent chocolate mint truffle made with fresh mint. (The girl behind the counter described it perfectly: you'll feel like there's a fresh mint leaf in your mouth.) Lovely but expensive.
Soma (Distillery District) is a favorite on the board, but I'm not sure about their mint offerings. You might try calling. http://www.somachocolate.com/SOMA_home.html
Xococava (north of St. Clair) is also worth a try: http://www.xococava.ca/
I really like Stubbe (Dupont and Christie, accessible by 26 bus from St. George stn or walk north from Christie stn), especially because their prices are quite reasonable for what you get. Not truffles but their mint praline is nice. You choose your own chocolates so you can mix and match with truffles as well. http://www.stubbechocolates.com/choco...
375 Dalhousie St, Ottawa, ON K1N, CA
99 Yorkville Avenue, Toronto, ON M5R 3K3, CA
I agree with the recommendations for soma and xococava. Not sure of their mint based offerings, but they both offer great chocolates with several interesting flavours. I also enjoy the hot chocolate they each offer. Soma also has some great cookies - haven't had as much success with the baked goods at xococava.
Another place I enjoy is Delight chocolate. They have two locations, one near Dundas and High Park (there are several buses that can get you there, from Dundas West/Keele/High Park/Kipling stations) and the other one on Queen near Bathurst (streetcar). Again, not sure about the mint, but they have nice organic, fair trade chocolates. I've enjoyed their ice cream a few times too!