Preheat oven to 350 degrees. Spray a 9 x 13 pan lightly with non stick cooking spray, line with parchment paper and spray again with more nonstick cooking spray. Set aside.
In large pot over medium low heat, melt butter, oil, chocolate and cocoa together until fully melted and smooth. Remove from heat.
Pour in sugar and vanilla extract. Add in eggs, 1 at a time, stirring vigorously before the next addition. After all four eggs have been mixed in, the batter should be thick and smooth without that granular look to it. Add in the remaining dry ingredients and stir gently until just combined. Do not over mix.
Pour batter into prepared pan. Smooth the top and bake 30-35 minutes or until toothpick comes out mostly clean with a few moist crumbs. Cool to room temperature.
For the mint filling: measure all ingredients into a large bowl and using an electric hand mixer, stir until thick and smooth. Add milk as needed to get a thick mixture. Spread over cooled brownies as evenly as possible and refrigerate.
For the chocolate topping: place chocolate chips and shortening into a small bowl and microwave until melted, stirring every 20 seconds. Pour over mint layer and spread until smooth. Refrigerate 30 minutes.
Remove bars from glass pan using the edges of the parchment to pull out completely. Using a very sharp knife, cut off the crispy edges and cut into 50-60 small squares. Serve.
Can be stored in an air tight container in the refrigerator for up to 4 days, though I doubt they will last that long!