Step 1

Toast the walnuts in a medium saute pan over medium heat, tossing frequently, until darkened and fragrant, about 5 minutes. Transfer to a cutting board to cool, then roughly chop.

Step 2

Line an 8-inch square cake pan with foil, leaving a 2-inch overhang on 2 sides. Coat lightly with cooking spray.

Step 3

Combine the chocolate and condensed milk in a large saucepan over medium-low heat. Cook, stirring frequently, until the chocolate is melted and the mixture is thick and smooth, 2 to 3 minutes. Remove from the heat. Add the marshmallow creme and butter and stir until completely combined and no streaks remain. Stir in the walnuts until evenly dispersed.

Step 4

Transfer the mixture to the prepared cake pan. Smooth with a rubber spatula into an even layer. Refrigerate until firm, 2 to 3 hours.

Step 5

Use the foil overhang to remove the fudge from the pan. Cut into small squares. The fudge can be refrigerated in an airtight container up to 5 days.