Heat oven to 350℉/177℃.
Spray a 9-inch square pan with baking spray. Set aside.
Melt butter and chocolate chips in a small saucepan over low heat, stirring constantly.
Remove from heat and set aside.
In a small bowl, whisk together flour and cocoa powder. Set aside.
In the bowl of an electric mixer, combine eggs, sugar, vanilla, and peppermint oil.
Beat on high speed for 10 minutes.
Stir in melted chocolate mixture.
Then slowly mix in flour mixture.
Pour into prepared pan.
Bake for 30 minutes, or until desired degree of doneness.