1. Make the cherry-port sauce: Bring port and granulated sugar to a boil in a small saucepan. Boil until reduced by one third, about 5 minutes. Stir in cherries. Simmer for 2 minutes. Remove from heat. Let stand, covered, until plump, about 10 minutes. Refrigerate until thick, about 40 minutes.

2. Meanwhile, make the semifreddo: Whisk mascarpone, cream, confectioners' sugar, and a pinch of salt until medium-stiff peaks form. Fold in chocolate. Spread mixture into a 5-by-9-inch loaf pan; cover with plastic wrap. Freeze until edges begin to harden and semifreddo is very cold, at least 50 minutes. Scoop into 4 serving dishes. Spoon cherry-port sauce over tops. Serve with chocolate wafer cookies.