Preheat oven to 325°F. Line an 8 or 9-inch square pan with parchment paper.
Melt the butter in a large saucepan, over medium heat. When the butter has melted, add 1 cup semi-sweet chocolate chips and stir until they have melted. Remove from the heat, add the sugar and stir to combine.
Add the vanilla extract and the eggs. Whisk or stir until smooth and shiny. Add the flour and salt and stir to combine. Allow the batter to cool before stirring in the mints.
Set aside 12 mints for topping the brownies, and chop the remaining mints in half. Stir the chopped mints into the batter, along with the reserved chocolate chips. Spread the batter into the bottom of the prepared pan and then place the remaining mints on top of the batter.
Bake for 28-30 minutes, or until an inserted toothpick shows moist crumbs. Let the brownies cool completely before slicing.
Notes
Allow the batter to cool before stirring in the mints. Wait at least 15 minutes. If you stir them in too early, they will melt into the batter. It will still be delicious, but it won't be as picture worthy or with distinct mint pieces in every bite.