Coat the inside of a 9 inch springform pan with cake release. (Link to our favorite pan release recipe in notes below).
In a large mixing bowl combine the granulated sugar, brown sugar, vanilla, salt, whole eggs and yolks.
In a microwave safe bowl melt the butter and chocolate together in 30 second increments, stirring in between, until melted and smooth. Whisk vegetable oil into the chocolate mixture.
Pour the chocolate mixture into the sugar mixture and mix until fully combined. Gently fold in the flour and cocoa.
Pour the brownie batter into the springform and bake at 350° F until a toothpick inserted in the center comes out clean (about 35 minutes). Remove from the oven, run a knife around the edge to loosen, and allow to cool in the pan for 30 minutes.
After 30 minutes of cool time, remove the springform edge and continue to let the brownies cool completely.
BUTTERCREAM
In a large mixing bowl whip the butter until fluffy. Add vanilla extract, and begin beating in powdered sugar in small increments.
When all sugar is incorporated, add milk 1 tablespoon at a time until the frosting is a pipeable consistency, (usually about 3-4 tablespoons total). Add in your food coloring until you reach your desired shade of green.
Cut the cooled brownies into 8 even triangle shaped slices (try using a plastic knife for clean slices). Pipe frosting with a star tip, then decorate as desired.
Notes
Calories are automatically generated. For best results calculate based on your exact ingredients.
If you don't have a star tip you can use a sandwich bag with the end snipped off.
You can flavor the buttercream by using your favorite coffee creamer instead of milk. I love using the peppermint mocha flavor for my butter cream.
You can use a double boiler to melt the butter and chocolate if you wish.