Line an 8x8 inch baking pan with parchment paper, leaving some overhang on two sides so you can lift the fudge out later. Set aside.

Toast the walnuts first if you have not already. Spread them in a dry skillet over medium heat and stir for 3 to 4 minutes until they smell nutty and turn a shade darker. Pour them onto a plate to cool. Toasting is worth the extra few minutes because it deepens their flavor noticeably.

In a medium heavy-bottomed saucepan over medium-low heat, combine the white chocolate chips, sweetened condensed milk, maple syrup, and butter. Stir constantly with a rubber spatula, scraping the bottom and sides so nothing scorches. The mixture will look clumpy at first, then smooth out as everything melts together, about 8 to 10 minutes. Keep the heat medium-low. High heat makes white chocolate seize up and turn grainy.

Once the mixture is completely smooth and glossy, remove the pan from heat. Stir in the salt and vanilla. Fold in the toasted walnuts, making sure they are distributed evenly throughout.

Pour the fudge into the prepared pan and spread it into an even layer with your spatula. Tap the pan gently on the counter a couple of times to settle it. If you want flaky sea salt on top, sprinkle it now.

Refrigerate uncovered for at least 2 hours, or until the fudge is completely firm when you press the center. Once firm, lift it out using the parchment overhang and set it on a cutting board.

Cut into small squares, about 1 inch each. This fudge is rich, so smaller pieces are better than big ones. A sharp knife drawn straight down, rather than dragged, gives you the cleanest cuts.