Preheat the oven to 325°. Line an 8-inch square pan with aluminum foil, letting the foil extend over two sides.
Begin by melting the chocolate chips for the cheesecake: Put the chips in a small bowl and microwave it, stirring every 15 seconds, until the chips are just melted. Set aside to cool.
Next, make the crust: Put the walnuts and wafers in the food processor and process until finely ground. Stir in the sugar and butter. Press the mixture on to the bottom of the pan.
Next, make the cheesecake: In an electric mixer, beat together the cream cheese, sugar, and vanilla. Then, beat in the sour cream. Beat in the eggs, one at a time. Stir in the melted chocolate. Do not overmix. Pour the mixture over the crust. Bake 40 minutes, or until the center is just a bit unset.
Cool the pan on a wire rack, until the bars are room temperature.
For the Sour Cream Topping: In a small bowl, stir together the sour cream, sugar, and vanilla. Spread it evenly over the bars.
For the Chocolate Drizzle: Melt the chocolate in the microwave, as above. Using a spoon, drizzle on the chocolate. I used any leftover chocolate to drizzle on to the serving plates. Refrigerate four hours or overnight. Peel off the foil and cut into 8 bars.