How should I add mint to my beer?

What about sanitizing it then muddling it, than add it to your bottling bucket and pour your fermented beer over it. Similar to using mint for drinks in a bar the oils will break down and stay in bottles, or you could boil the mint in your priming sugar for boiling and make a simple syrup.

Adding mint during the boil is good, but the problem is that you'll lose a lot of the mint aroma during primary fermentation as the C02 carries it out the airlock.

The first thing that comes to mind would be to create a mint extract (soak the mint in vodka) and add that to your secondary. You can, of course, add the mint directly to the secondary, but you won't see as much flavor imparted as you would with an extract, and you'd need to remove it after a week or so before the leaves break down (resulting in potential grassy off-flavors).

Considering how well peppermint pairs with dark chocolate, I'd be inclined to use that over something like spearmint.

Have you considered making an extract from your mint using vodka? It would give you a lot of control over how minty your stout ends up. When you're ready to bottle you can take a small sample of the beer and add the mint extract until the flavor profile is what you're looking for. Then you just scale the amount up for the whole batch.