Toast the walnuts: Preheat the oven to 350ºF. Arrange the walnuts in a single layer on a rimmed baking sheet lined with parchment paper or foil. Toast for 7 to 10 minutes, stirring a few times during baking, until the nuts are lightly browned and fragrant. Because the walnuts around the edges toast faster than those in the center, give them a quick stir to help them brown evenly. Once they’re done, transfer the walnuts to a plate right away so they can cool and don’t continue to toast on the hot pan. Once the walnuts cool, rough chop them, dividing them into 1 cup and ½ cup portions. You can also skip toasting and use raw walnuts.

Line an 8×8 or 9×9 inch square pan with aluminum foil, allowing some overhang so you can easily lift the fudge out later. Set aside.

Add the maple syrup to a small saucepan over medium heat. Once it comes to a boil, reduce the heat to medium low or low and let it gently boil for about 15 to 20 minutes, keeping an eye on it as it reduces to make sure it doesn’t overflow. If the bubbles are getting close to the edge of the pan, turn the heat down. After about 15 minutes, carefully pour the syrup into a measuring cup to check the volume. If it has not reduced to about ½ cup yet, return it to the pan and continue simmering. Once reduced, set the syrup aside to cool while you prepare the other ingredients. As it cools, the syrup will crystallize and no longer be liquid. This is normal.

In a medium saucepan over medium heat, combine the granulated sugar, brown sugar, butter, evaporated milk, corn syrup, and the reduced maple syrup. Stir constantly until the sugar is melted. Then stir occasionally until the mixture comes to a full boil.

Do not stir. Let the mixture continue boiling for 4 to 8 minutes, or until an instant-read thermometer reaches 236 to 238ºF. Be sure the thermometer tip is suspended in the liquid and not touching the bottom of the pan, which can give a false reading.

Remove the saucepan from the heat and immediately stir in the white chocolate chips, marshmallow creme, and salt. Stir until the mixture is smooth and completely melted.

Stir in 1 cup of toasted walnuts, mixing until evenly distributed throughout the fudge.

Pour the fudge into the prepared pan. Sprinkle the remaining ½ cup of walnuts on top.

Allow the fudge to cool and fully set before cutting, about 6 to 8 hours, though overnight is even better for clean slices.