Place chocolate chips and butter in heavy 2-quart saucepan. Place over low heat.

Cook, stirring constantly, until chocolate has melted. Remove from heat.

Stir in whipping cream.

Place in refrigerator to cool for 30 - 40 minutes, stirring frequently, just until thick enough to mound and hold its shape when dropped from a spoon. (If filling becomes too thick, microwave on High (100%) 10 - 15 seconds to soften.)