Stir together 5 (1-oz.) semisweet chocolate baking squares and 1/2 cup butter in a heavy saucepan over medium-low heat, whisking constantly until melted. Whisk in 1 cup evaporated milk until blended. Gradually whisk in 1 (16-oz.) box powdered sugar; stir constantly until blended and smooth, and simmer 1 minute. Serve warm. Sauce may be stored in an airtight container in the refrigerator up to 2 weeks.