Heat oven to 350°F. Line 13x9x2-inch baking pan with foil, extending foil beyond edges of pan. Grease foil.
Stir together butter, sugar and vanilla in large bowl with spoon or wire whisk. Add eggs; stir until well blended. Stir in flour, cocoa, baking powder and salt; blend well. Spread in prepared pan.
Bake 20 to 25 minutes or until brownie starts to pull away from side of pan. Meanwhile make cheesecake layer.
Immediately after removing brownie from oven, place KIT KAT® Bars over brownie surface. Spoon cheesecake mixture over bars; spread evenly. Return to oven; continue baking 35 to 40 minutes or until almost set. Remove from oven to wire rack. With knife, loosen cake from side of pan. Cool 2 hours. Cover; refrigerate 3 to 4 hours or until bars are thoroughly chilled. Cut into bars. Allow bars to soften slightly before serving. Cover; store leftover bars in refrigerator. Makes about 24 bars.
Cheesecake layer: Beat 3 packages (8 oz. each) softened cream cheese and 3/4 cup sugar until smooth in large mixer bowl. Add 3 eggs, one at a time, beating well after each addition. Stir in 1 teaspoon vanilla extract.