Line 8-inch square pan with foil, allowing foil to extend over sides of pan. Spray with no stick cooking spray. Mix sugar and cocoa powder in large bowl. Set aside.
Place butter and 1/4 cup milk in medium microwavable bowl. Microwave on HIGH 1 minute; stir until butter is melted. Stir in vanilla. Add to sugar mixture; mix well. Pour into prepared pan. Refrigerate at least 1 hour or until firm.
For the mint layer, place white chocolate chips and 2 tablespoons milk in medium microwavable bowl. Microwave on MEDIUM 1 minute; stir. Microwave additional 15 to 30 seconds, stirring after 15 seconds. Stir in mint extract and green food color. Spread over cooled chocolate layer.
Refrigerate 10 to 15 minutes or until firm. Use foil to lift out of pan onto cutting board. Cut into 16 (2-inch) squares. Cut each square diagonally in half, making triangles.