Line an 8x8-inch square baking dish with parchment paper, leaving some overhang on the sides for easy removal. Lightly butter the paper. Get a candy thermometer ready if you have one—it helps with precision.
In a medium heavy-bottomed saucepan, combine the maple syrup, heavy cream, butter, and salt. Stir over medium heat until the butter melts. Bring the mixture to a gentle boil and continue cooking, without stirring, until it reaches 238°F (soft-ball stage) on a candy thermometer. This usually takes about 15–20 minutes.
Remove the pan from heat and let it cool undisturbed for 5 minutes. Add the vanilla extract. Using a hand mixer or wooden spoon, beat the mixture vigorously for 5–8 minutes until it thickens, loses its gloss, and begins to hold its shape. If using nuts, fold them in at this stage.
Quickly pour the fudge into the prepared pan and spread it evenly. Let it cool at room temperature for at least 2 hours or until firm. Once set, lift out using the parchment paper and cut into bite-sized squares.