1For crust, generously butter a 9 inch springform pan. Break chocolate wafers into a food processor and pulse til fine crumbs form. Add melted butter and pulse just til blended. Press crumb mixture onto bottom and 2 inches up sides of prepared pan. Refrigerate. Can be made a day ahead.
2For Chocolate, Preheat oven to 325. Position oven rack in lower third of oven. Place 1 1/4 cups mint chocolate chips together with water in microwave safe bowl; microwave on medium setting til melted, about 1 1/2 minutes in 30 seconds increments. Blend in creme de menthe. Spoon chocolate mixture over bottom of crust, using back of a spoon to spread evenly. Set aside.
3For filling, Put cream cheese in large bowl. Using a mixer, mix for 30 seconds till smooth and creamy. Add sugar in a steady steam, mixing for about 1 1/2 minutes. Mixing on low speed, add eggs, vanilla and sour cream, blending well. Pour filling over chocolate4, starting around the sides and then filling in the middle. Bake for 50 minutes. At the end of 50 minutes, turn off oven and leave cake in oven for another hour without opening oven door. Remove form oven. Chop remaining 1/4 cup chocolate mint chips and sprinkle evenly on surface of warm cake. Cool on rack for three hours, then refrigerate for at least 24 hours before serving. To serve, run a think knife around edge of pan and carefully remove the rim. Serve in wedges. Serves 10 to 12