Grease an 8x8-inch square baking dish with butter.
Place sweetened condensed milk, chocolate chips, and baking chocolate in a microwave-safe dish; cook in microwave on low power until chocolate has melted, about 3 minutes. Beat with an electric mixer until smooth, about 1 minute.
Pour fudge into the prepared baking dish; let stand until set, 3 to 4 hours. Cut into squares to serve.