Melt butter and unsweetened chocolate squares in a 3-quart heavy saucepan over low heat, stirring constantly. Remove from heat, and cool 5 minutes. Stir in 1 1/2 cups sugar. Add 2 eggs, 1 at a time, blending well after each addition. Add milk and 1 teaspoon vanilla. Add flour and salt; stirring until well blended. Spread mixture evenly into a lightly greased 9-inch springform pan.
Bake at 325° for 25 minutes. Remove from oven, and cool in pan on a wire rack. Reduce oven temperature to 300°.
Beat cream cheese and remaining 3/4 cup sugar at medium speed with an electric mixer until smooth. Add remaining 1 teaspoon vanilla, beating just until blended. Add remaining 3 eggs, 1 at a time, beating at low speed just until blended after each addition. Add sour cream and melted semisweet chocolate squares, beating just until blended.
Spoon cherry pie filling evenly over prepared crust. Pour chocolate cheesecake mixture over cherry pie filling.
Bake at 300° for 1 hour and 5 minutes to 1 hour and 10 minutes or until edges of cheesecake are set. (Center of cheesecake will not appear set.) Turn oven off; let cheesecake stand in oven 15 minutes. Remove cheesecake from oven; gently run a knife around edge of cheesecake to loosen. Cool completely on a wire rack. Cover and chill 8 hours. Release and remove sides of pan; serve with desired toppings.