TheFork users who ate in Cognac have said really good things about Chez Bianca et Philippe, Hotel Chais Monnet - La Distillerie and La Belle Epoque. Maybe you should give them a try!
Some of the more popular Salad restaurants having a terrace in Cognac according to TheFork users include:
Chez Bianca et Philippe, with a 9.3 rating
Hotel Chais Monnet - La Distillerie, with a 9 rating
La Belle Epoque, with a 9.2 rating