Mix sugar, maple syrup, milk and salt in a large saucepan. Cook over low heat (without stirring) until the sugar dissolves and the mixture comes to a boil.
Cook mixture until it reaches 234°F to 240°F (112°C to 116°C) on a candy thermometer, or until a teaspoonful forms a soft, flexible ball when dropped into cold water. Remove pan from heat and add butter. Do not stir. Let fudge cool to 110°F (43°C), or until the bottom of the pan feels barely warm to the touch, 30 to 45 min.
Stir in vanilla and beat fudge with a wooden spoon until it is slightly thickened. Add walnuts and continue beating until fudge begins to lose its glossy finish. Quickly spread fudge evenly into an oiled or parchment paper-lined 8 in. (2 L) square pan. Cool completely before cutting into squares. Store in an airtight container in the fridge for up to 1 week.