Prepare crumb crust by combining cracker crumbs, butter, and 2 Tbsp. sugar. Press mixture into bottom and 2 inches up sides of a 9-inch springform pan. Refrigerate.
Melt chocolate chips in top of a double boiler over hot water. Set aside.
Beat cream cheese and 1 cup sugar in a large mixing bowl until smooth and creamy. Blend in eggs. Gradually pour in chocolate mixture, beating on low speed until well blended. Add 1/2 cup sour cream, flour, and vanilla. Blend until smooth. Pour into prepared crust.
Bake in a preheated 350-degree oven to 55-60 minutes or until filling is firm. Turn oven off. Cool cheesecake 1 hour without opening door. Cool completely. Chill several hours or overnight.
Garnish just before serving with sweetened sour cream made by combining 1 Tbsp. sugar with 1/2 cup sour cream.