Preheat oven to 350. Line an 8×8 pan with parchment (or aluminum foil). Combine sugar, flour, cocoa, baking powder, and salt. In a separate bowl, whisk oil, eggs, and vanilla. Add flour mixture and stir gently to combine Stir gently to combine. Spoon into prepared baking pan. Bake until toothpick comes out clean, about 25 to 30 minutes. Cool completely before proceeding.

To make the mint layer:

Use an electric mixer to beat butter until light and fluffy. Add powdered sugar a little at a time until it reaches buttercream consistency. Add extracts and food coloring. Spread over brownies. Place in refrigerator (or freezer if pressed for time) until hardened.

Put butter and milk in a small saucepan. Once the butter has melted add chocolate and whisk vigorously until melted. Remove from heat and add vanilla. Let cool for about 5 minutes. Use an electric mixer to add powdered sugar. Spread over mint layer. Place in refrigerator to harden slightly before slicing.