Preheat oven to 350 degrees Fahrenheit. Prepare a 9×13 inch baking pan with either butter or parchment paper.
Melt butter in a small saucepan over medium heat. Remove from heat, add chocolate and let sit for two minutes. Stir chocolate and butter mixture until smooth. Set aside to cool.
In the bowl of an electric mixer fitted with the whisk attachment, beat eggs, brown sugar, salt and vanilla extract on high for 5 minutes until thick and pale. Mixture should fall from the whisk in a thick ribbon and dissolve back into the surface slowly.
Using a large rubber spatula, fold cooled chocolate into egg mixture until fully combined.
Gradually fold in flour and ½ cup mini melt away mints, mixing until just combined. Avoid over mixing so that the batter remains thick and fluffily.
Immediately pour batter into prepared baking dish and bake for 20 minutes. Remove from oven and sprinkle remaining ½ cup mints over the top.
Return to oven and bake an additional 10-15 minutes until the top begins to crack and there is just a slight wiggle left to the batter.
Allow to cool completely on a wire rack before cutting into squares. Store in an airtight container at room temperature.