In a large microwave-safe bowl, combine butter and chocolate. Cover and microwave on high for 30-60 seconds; stir until smooth. Beat in sugar, eggs and vanilla. Combine flour and baking powder; gradually add to chocolate mixture until blended. Spread into a greased microwave-safe 8-in. square dish; set aside.
In a large microwave-safe bowl, beat cream cheese until fluffy. Beat in the sugar, egg and vanilla until smooth. Spoon over brownie batter; cut through batter with a knife to swirl. Sprinkle with chocolate chips.
Cook, uncovered, at 70% power for 8-10 minutes or until a toothpick comes out clean. Cook on high for 1 minute longer. Remove to a wire rack to cool completely. Store in the refrigerator.