Microwave Peanut Butter Fudge

1/2 cup (1 stick) butter or margarine
2/3 cup chunky peanut butter
4 1/2 cups confectioners' sugar
1/3 cup milk
1 tablespoon vanilla extract

Place butter and peanut butter in a large bowl. Microwave on HIGH until the butter melts (1 to 1 1/2 minutes). Stir until blended. Add remaining ingredients; stir until lumps of sugar disappear. Microwave on HIGH until softened but not bubbly (1 to 1 1/2 minutes); stir. Pour into buttered 9-inch square baking pan. Cover, and refrigerate for at least one hour.

Store refrigerated.