1 (20 ounce) package brownie mix
4 (8 ounce) packages PHILADELPHIA Cream Cheese, softened
1 cup sugar
1 teaspoon vanilla
1/2 cup BREAKSTONE'S or KNUDSEN Sour Cream
3 eggs
2 (1 ounce) squares BAKER'S Semi-Sweet Baking Chocolate

Preheat oven to 325 degrees F. Spray 13x9-inch baking pan with cooking spray. Prepare brownie batter as directed on package; pour into prepared pan. Bake 25 min. or until top of brownie is shiny and center is almost set.
Meanwhile, beat cream cheese, sugar and vanilla in large bowl with electric mixer on medium speed until well blended. Add sour cream; mix well. Add eggs, one at a time, mixing on low speed after each addition just until blended. Gently pour over brownie layer in pan. (Filling will come almost to top of pan.)
Bake 40 min. or until center is almost set. Run knife or metal spatula around rim of pan to loosen side of dessert from pan; cool. Refrigerate at least 4 hours or overnight.
Melt chocolate as directed on package; drizzle over cheesecake. Refrigerate 15 min. or until chocolate is firm. Cut cheesecake into 16 pieces to serve. Store any leftover cheesecake in refrigerator.