Preheat oven to 350 degrees. Line an 8 inch baking dish with foil or parchment paper. Set aside.
In a bowl mix together melted chocolate and vegetable oil until smooth. Add eggs and sugar, and mix until well combined. Fold in flour, baking powder and salt Pour into prepared baking dish.
Bake for 20-25 minutes until a tooth pick comes out mostly clean.
Remove from oven and cool completely.
Meanwhile, mix together the buttercream.
In a bowl mix together butter, sugar, mint extract and milk until you get the desired consistency. Add more sugar or milk if necessary. Add a few drops of green food coloring, mix until you get the desired color.
Frost cooled brownies.
In a microwave safe bowl heat chocolate and heavy cream for 45 seconds. Stir until smooth and well combined. Pour over frosting.
Chill brownies until the chocolate sets. Slice to serve.