Line a 9x13-inch baking pan with parchment paper or aluminum foil, leaving an overhang on the sides for easy removal. Lightly butter the lining.
In a large heavy-bottomed saucepan, combine sugar, butter, evaporated milk, and maple syrup. Bring to a full rolling boil over medium heat, stirring constantly.
Continue boiling for 5 minutes, stirring constantly to prevent burning. Remove from heat.
Quickly stir in marshmallow creme, white chocolate chips, vanilla, and salt until completely smooth and chocolate is melted.
Fold in toasted walnuts and immediately pour into prepared pan. Spread evenly with a spatula.
Let cool completely at room temperature for at least 3 hours, or refrigerate for 1 hour until firm.
Using the parchment overhang, lift fudge from pan and cut into 1-inch squares. Store in an airtight container with parchment between layers for up to 2 weeks.