Process 1 (12-oz.) package frozen raspberries, thawed, in a blender until smooth, stopping to scrape down sides. Pour through a fine wire-mesh strainer into a bowl, pressing mixture with the back of a spoon. Discard seeds. Stir in 3 Tbsp. powdered sugar until blended. Sauce may be stored in an airtight container in the refrigerator up to 2 weeks.