Prepare crumb crust. Then combine first 6 ingredients in a medium saucepan, whisking well. Bring to a boil over medium heat, whisk constantly for 8 minutes. Remove from heat.
Stir in chocolate until smooth. Stir in butter and vanilla. Pour into pastry shell. Cover top of tart with wax paper. Chill at least 3 hours.
Whip cream and sugar until stiff. Spoon on top of tart.