Line a small 8x4-inch loaf pan with parchment paper, leaving overhang on the sides for easy removal.
In a microwave-safe bowl, combine the sugar-free white chocolate chips and butter. Microwave in 30-second intervals, stirring between each one, until completely melted and smooth.
Add the softened cream cheese, maple extract, powdered erythritol, and a pinch of sea salt to the melted chocolate mixture. Stir vigorously until everything is fully combined and the batter is glossy and smooth.
Fold in the chopped walnuts, reserving about 2 tablespoons for topping.
Pour the fudge mixture into the prepared loaf pan and spread it evenly with a spatula. Press the reserved walnuts gently into the top.
Refrigerate for at least 2 hours until the fudge is completely firm. Lift out using the parchment overhang, slice into 12 pieces, and store in the refrigerator for up to 10 days.