In double boiler set over hot simmering water, heat chocolate chips 2 to 3 minutes, stirring frequently, until melted and smooth. Gradually add 1 cup whipping cream, stirring constantly, until combined. Stir in vanilla and egg yolks until well blended. Cook over medium-low heat 5 to 6 minutes, stirring frequently, until thickened and hot. Pour filling into crust. Refrigerate at least 3 hours or until firm.