In small bowl, mix cookie crumbs and butter. In ungreased 9-inch glass pie plate, press crumb mixture in bottom and 1 inch up side.
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.
In medium bowl, beat 1/2 cup whipping cream and the powdered sugar with electric mixer on high speed 1 to 2 minutes or until soft peaks form. Top individual servings with whipped cream. Dust with cocoa.