In a heavy saucepan (or double-boiler), dissolve cornstarch in water, stir in sweetened condensed milk, egg yolks and German chocolate. Cook and stir over medium heat until thickened and bubbly. Remove from heat, add butter, vanilla and coconut. Cool slightly. Arrange mini chocolate chips on bottom of prepared pastry shell. Pour filling over chocolate chips and allow to cool. Cover and refrigerate 4 hours or until set. Serve topped with whipped cream.