Line and 8-inch square baking pan with foil, leaving at least a 1-inch overhang on all sides. Mist with cooking spray. Combine brown sugar, marshmallow fluff, evaporated milk, butter and salt in a heavy saucepan. Cook over medium heat, whisking often, until it comes to a boil. Let it boil for 5 minutes, shirting consistently to prevent scorching. Remove pan from heat; stir in white chocolate chips until fully melted and mixture is smooth. Stir in walnuts and maple extract. Quickly pour into baking pan and smooth top. Refrigerate uncovered until firm, (about 2 hours).