Fresh in-season raspberries are essential to this ruby-hued sauce. It will keep five days tightly covered in the refrigerator.
Combine first 3 ingredients and, if desired, orange liqueur, in a large saucepan. Bring to a boil; reduce heat, and simmer, uncovered, 5 minutes or just until raspberry mixture starts to thicken. Remove from heat, and let cool completely.
Press raspberry mixture through a wire-mesh strainer into a small saucepan, using the back of a spoon to squeeze out juice. Discard pulp and seeds. Whisk in 1 tsp. cornstarch. Bring to a boil, and cook 1 to 2 minutes or until thickened. Store in refrigerator up to 3 days.