Liberally grease 13X9 pan. (I use a slightly smaller pan so the brownies come out more moist!) Preheat oven to 350. In a double boiler, or over low heat, melt butter and 4 oz chocolate and cool slightly.
In a large bowl whisk eggs and add sugars, salt, coco powder, melted chocolate mixture, and vanilla. Fold in flour until just combined then add Andes Mints.
Spread batter in pan and bake 35-40 minutes, or until top is shiny and beginning to crack. Let cool in pan on wire rack before cutting.