Brownies: Combine sugars and butter. Add eggs and vanilla, then cocoa. Mix in flour and salt. Stir in chocolate chips and marshmallows. Pour into a greased 9x13 pan. Bake at 350 degrees for 25-28 minutes, or until well done. DO NOT overbake. Let cool to room temperature.
Buttercream: Beat the butter and sugar together, then slowly add the milk while you continue beating, until you get a frosting consistency Mix in the extract and salt, then the food coloring. You can add more less of these last three to taste, and to the desired shade of green. Generously frost the brownies once they're cooled.
Chocolate Ganache: Add chocolate chips and heavy cream to a microwave-safe glass bowl. Microwave for 30 seconds, stir, then cook in 10 second intervals until you can stir it smooth. Immediately pour on top of the frosting, and spread to cover. Refrigerate until the chocolate is firm.