Combine graham cracker crumbs, sugar, walnuts and melted butter in small bowl. Mix well and press firmly onto bottom of a 9" springform pan. Chill 30 minutes.
Preheat oven to 325 degrees.
Beat cream cheese in large bowl until fluffy. Gradually beat in sugar and blend well. Add eggs, 1 at a time, beating well after each addition. Beat in flour and vanilla, then stir in chocolate chips.
Pour filling into chilled crust.
Bake until edges are set and light golden, about 50-60 minutes. The center should be just set but not completely firm. Remove from oven and place on rack - maintain oven temperature.
In a small bowl mix together sour cream, sugar and vanilla extract. Pour on cheesecake and smooth top if necessary. Return to oven and bake 5-7 minutes until just set. Cool cheesecake on rack for 1 hour, then chill in pan overnight.
Heat cream in a small saucepan until simmering. Add chocolate chips and remove from heat. Whisk until smooth and cool slightly.
Remove side of pan from cheesecake and place on serving platter.
Pour glaze over cold cheesecake and smooth with a spatula if necessary. Chill briefly to set glaze. Garnish with walnut halves if desired.