Combine graham cracker crumbs, 1/4 cup sugar, cocoa and butter or margarine. Mix well. Press mixture into bottom of a 9-inch spring form pan. Bake at 325 degrees for 5 minutes. Cool. Beat cream cheese with mixer until light and fluffy; gradually add 3/4 cup sugar, mixing well. Beat in 1/2 cup cocoa. Add eggs, one at a time, beating well after each addition. Stir in coffee, Kahlua and 1 teaspoon vanilla extract. Pour into prepared pan. Bake at 375 degrees for 25 minutes. Combine sour cream, 2 tablespoons sugar and 1 teaspoon vanilla; spread over hot cheesecake. Bake 425 degrees for 5 to 7 minutes. Let cool at room temperature on a wire rack. Chill 8 hours or overnight. Remove side of spring form pan. To garnish, place 3 chocolate curls in the center of cheesecake; gently break the remaining chocolate curls and sprinkle over cheesecake if desired.