PREHEAT oven to 325°F. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Add 1-1/2 cups of the sugar; mix well.
ADD 2 of the eggs, one at a time, mixing on low speed after each addition just until blended. Add milk and 1 tsp. of the vanilla; mix well. Combine flour and salt. Gradually add to chocolate mixture, mixing after each addition just until blended. Spread evenly into greased and floured 9-inch springform pan. Bake 25 min.
BEAT cream cheese, remaining 3/4 cup sugar and remaining 1 tsp. vanilla in large bowl with electric mixer on medium speed until well blended. Add sour cream; mix well. Add remaining 3 eggs, one at a time, mixing on low speed after each addition just until blended. Pour over brownie layer in pan (Filling will come almost to top of pan.)
BAKE 55 min. to 1 hour or until center is almost set. Run knife or metal spatula around rim of pan to loosen cake; cool before removing rim of pan. Refrigerate 4 hours or overnight. Let stand at room temperature 30 min. before serving. Store leftover cheesecake in refrigerator.