Heat oven to 350°F Spray 9-inch glass pie plate with cooking spray. In 3-quart saucepan, heat milk, butter and chocolate over medium heat 5 to 7 minutes, stirring frequently, just until chocolate is melted; remove from heat.
In medium bowl, stir chocolate mixture and remaining ingredients except whipped topping with wire whisk or fork until smooth. Pour into pie plate.
Bake 30 to 35 minutes or knife inserted in center comes out clean. Cool 30 minutes. Refrigerate at least 2 hours or until chilled. Garnish with whipped topping. Store in refrigerator.