I did a mint chocolate stout two years back as part if a stout experiment (split 5 gal of stout into one gal jugs and added dif flavors to each).
I muddled and chopped up fresh mint leaves, and then soaked them and cacao nibs in vodka for one week, then pitched it all into one of the gal jugs with the stout and let it sit one more week before bottling.
Turned out really tasty. Slightly astringent from the leaves being in there for a week, I think, but that didn't detract too much from the overall flavor, IMO... Though next time I might only pitch the vodka extract without the leaves and see what that tastes like. I'd go this route, if I were you.
I don't recall off the top if my head how much mint I used (1/4 oz, maybe?) for the one gallon (not at home to reference my notes)... But if you want to know you can listen to the Basic Brewing Radio podcast from 3/6/14 - James interviewed me about the whole stout experiment on there, and I did discuss the recipes. [emoji482]