Brownie: Melt butter and chocolate in heavy pan over low heat, stirring constantly, cool. When cooled, add sugar and eggs, one at a time, mix well until blended after each addition. Blend in milk and vanilla. Stir in flour and salt, mixing till blended. Spoon into a greased and floured spring form pan, spreading evenly. Bake at 350F for 25 minutes.
Cheesecake: Beat cream cheese, sugar & vanilla at medium speed with an electric mixer until well blended. Add eggs 1 at a time, mixing at low speed. Add in sour cream and pour over brownie mixture that has been cooked. Bake 55 minutes or until center is almost set. Cool before removing from oven. Remove rim and garnish with white chocolate and dark chocolate curls.