Line a 9x13-inch pan with parchment or foil, leaving overhang; lightly butter lining.
Combine sugar, butter, evaporated milk, and maple syrup in a large heavy-bottomed saucepan; bring to a full rolling boil over medium, stirring constantly.
Boil 5 minutes, stirring constantly to prevent burning; remove from heat.
Quickly stir in marshmallow creme, white chocolate, vanilla, and salt until chocolate melts and mixture is smooth.
Fold in toasted walnuts; immediately pour into pan and spread evenly with a spatula.
Cool completely at room temperature at least 3 hours or refrigerate 1 hour until firm.
Lift out by parchment; cut into 1-inch squares and store airtight with parchment between layers up to 2 weeks.