In a bowl, beat cream cheese and sugar. Add eggs, yogurt, vanilla, lemon juice and biscuit mix; mix just until smooth. Pour into a greased 9-inch square baking dish. Bake at 350° for 25-30 minutes or until center is nearly set. Place on a wire rack while preparing topping. In a bowl, combine sour cream, sugar and vanilla until smooth. Carefully spread over cheesecake. Bake for 4 minutes (topping will not brown or set). Cool on a wire rack for 1 hour. Refrigerate until completely cooled. Garnish with fruit if desired.