In small bowl, combine wafer crumbs and sugar; stir in butter. Press onto the bottom and 1 1/2 inch up sides of a greased 9 inch springform pan. Bake at 350 for 10 minutes. Cool on a wire rack.
In microwave or heavy saucepan, melt chocolate chips; stir until smooth. Cool. In mixing bowl, combine egg yolks, cream and vanilla; beat well. Gradually stir a third of the cream mixture into melted chocolate until blended. Gradually stir in remaining cream mixture. Pour into crust.
Place pan on a baking sheet. Bake at 350 for 40-45 minutes or until center is almost set. Cool on a wire rack for 10 minutes. Carefully run a knife around edge of pan to loosen; cool 1 hour longer. Refrigerate overnight.
For frosting, in mixing bowl, cream butter. Combine confectioners sugar and cocoa; add butter with enough milk to achieve frosting consistency. Pipe into center of dessert, Refrigerate leftovers.