Graham Cracker Crumb Tarts:  Mix together the graham cracker crumbs, melted butter and sugar.  Evenly divide the mixture and press onto the bottom and up the sides of eight - 4 inch (10 cm) tart pans with removable bottoms.  (Each tart will use 2 - 3 tablespoons of the graham cracker crumb mixture.)  Place the  tart shells in the refrigerator to chill while you make the filling.

Cream Filling:  Melt the chopped chocolate in a heatproof bowl placed over a saucepan of simmering water. (Watch carefully as white chocolate scorches very easily.) Remove from heat and set aside.   In a large bowl, beat the cream cheese until fluffy and smooth (about 2 - 3 minutes).  Add the melted white chocolate and then the sour cream and vanilla extract, beating until you have a creamy smooth mixture.  Taste and add a little sugar, if desired.  Evenly divide the cream among the tart shells smoothing the tops with the back of a spoon or an offset spatula.  Cover and refrigerate until firm. 

To serve:  Top with fresh berries or cut up fruit.

Note:  You can make both the graham cracker tart shells and the filling a few days in advance. But to prevent the crust from softening, do not assemble the tarts until the day they are being served.

Makes about 8 - 4 inch (10 cm) tarts.

Note: Instead of individual tarts you can make one large 9-inch (23 cm) tart.