Place salmon on broiler pan, skin-side down.
In a small bowl, combine brown sugar, dill and mustard. Spread half on top of salmon. Cover with plastic wrap and refrigerate for up to 5 hours. Stir remaining mixture into mayonnaise and refrigerate sauce, covered, until ready to serve.
To serve, heat oven to 400 F. Remove salmon from refrigerator 20 minutes before cooking. Bake at 400 F for 20 minutes or until fish flakes easily with a fork. Remove salmon in one piece to a platter, leaving skin behind. Serve warm or at room temperature with sauce.