Grease the sides of a metal loaf pan with butter, then line the pan with parchment paper from long side to long side, leaving a little overhang.
In a large saucepan, bring the maple syrup to a boil over medium-high heat then lower to a simmer for 5 minutes.
When the syrup rises as it boils, simply lift the pan from the heat for a few seconds and swirl the pan a little to keep the syrup from scorching.
Pour in the heavy cream without stirring it in and bring the mixture back to a boil and simmer until it reaches 236°F on a candy thermometer*, about 20-25 minutes. The mixture will rise again so watch it!
Remove the pan from the heat and add the butter without stirring and set aside to cool for about 10 minutes.
Transfer the mixture to a mixing bowl and beat with the paddle attachment on an electric mixer on medium speed until the mixture thickens and just starts to lose it’s sheen, about 5 minutes.
Fold in Chopped Walnuts.
Pour the fudge into the prepared pan and spread out evenly with a spatula and refrigerate until completely set, about 2 hours.
Lift the fudge from the pan and cut into squares with a sharp knife.
