Preheat the oven to 350F (175C). Line the inside of a 9 by 13-inch, or similar sized baking pan, with foil. Butter or spray lightly with cooking spray.
In a large bowl set over a pan of simmering water, melt the chocolate and butter.
In a small bowl, whisk together the flour, salt, and cocoa powder.
Once the chocolate is melted and smooth, over the heat, whisk in both sugars. Remove from heat and whisk in three of the eggs completely, then whisk in the other two, along with the vanilla and mint extract, if using.
Sprinkle the flour mixture and the Altoids over the top and using a spatula, gently fold in the dry ingredients until just combined; there might be just a trace of the flour in places. Do not overmix.
Scrape into the prepared pan and bake for 30 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs attached. Like most brownie recipes, it’s best to err on the side of underbaked than over.