Prepare crumb crust. Then combine first 6 ingredients in a medium saucepan, whisking well to blend. Bring to a boil over medium heat, whisking constantly, for about 8 minutes. Remove from heat and stir in the butter and vanilla.
Place chopped chocolate in a small microwavable bowl. Microwave at medium power for 1 minute. Stir and repeat. Stir and cook the chocolate 30 seconds more if necessary, or until smooth.
Remove 1 cup of custard from saucepan and stir into melted chocolate. Spread chocolate custard evenly in bottom of pie shell. Spoon remaining vanilla custard on top. Cover top of tart with a sheet of wax paper to prevent skin from forming. Chill vanilla cream pie at least 3 hours or overnight.
To serve, whip cream and confectioners' sugar until stiff. Spoon or decoratively pipe whipped cream on top of tart.