2/3 cup butter or margarine
1 2/3 cups (10-ounce package) mint chocolate chips - divided use
1 1/2 cups granulated sugar
1 cup all-purpose flour
1/3 cup cocoa
1 teaspoon vanilla extract
1/2 teaspoon baking powder
1/2 teaspoon salt
3 large eggs
1 cup coarsely chopped walnuts
Preheat oven to 350°F (175°C). Grease 13 x 9 x 2-inch baking pan.
Place butter and 1 cup mint chocolate chips in large microwave-safe bowl. Microwave at HIGH (100%) 1 to 1 1/2 minutes or until chips are melted when stirred.
Add sugar, flour, cocoa, vanilla, baking powder, salt and eggs into the chocolate mixture; stir with spoon until smooth. Stir in remaining 2/3 cup chips. Spread batter into prepared pan; sprinkle walnuts over top.
Bake for 30 minutes or until center is set. Cool completely in pan on wire rack before cutting into bars.
Store Mint Chocolate Brownies in an air-tight container.