Brownies layer - Prepare a 9 x 13 baking dish with parchment paper. Spray pan with cooking spray then line with parchment paper both ways (to create an easy to remove grip). Preheat oven to 325 degrees F.
In a medium saucepan, melt butter. Coarsely chop chocolate bars and add to melted butter pan. Whisk chocolate mixture until smooth.
Remove saucepan from heat then stir in sugar and vanilla. Add eggs, one at a time, beating well after each addition.
Stir in flour and salt and mix until combined.
Bake in 325 degree F oven for 50 minutes or until toothpick inserted into the center comes out clean.
Cool brownies before frosting.
Mint Layer - In a large bowl, beat butter and salt with mixer. Slowly beat in a cup of confectioner's sugar at a time. Whip in green food coloring and mint extract. Slowly beat in milk and beat mixture until smooth.
Pour mixture over cooled brownie layer and evenly spread with a spatula. Refrigerate for 30 minutes to 1 hour.
Chocolate Topping - In a small saucepan over low heat, melt butter. Stir in chocolate chips and mix until melted and smooth. Pour chocolate topping over mint layer and evenly spread with a spatula.
Refrigerate brownie pan an chill for at least 2 hours.
Pull parchment paper sides out of pan and peel back; discard paper. Use a sharp knife dipped in hot water to cut clean squares.
I cut big squares then quartered each square into 4 to get 48.