Step 1

Make foil sling for 8-inch square baking pan by folding 2 long sheets of aluminum foil so each is 8 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Spray foil with vegetable oil spray.

Step 2

Combine sugar, butter, evaporated milk, and salt in large saucepan. Bring to boil over medium-high heat, stirring frequently. Once boiling, reduce heat to medium-low and simmer, stirring frequently, until mixture registers 234 degrees, 3 to 5 minutes.

Step 3

Off heat, add chocolate and marshmallows and whisk until smooth and all marshmallows are fully melted, about 2 minutes (fudge will thicken to consistency of frosting). Stir in walnuts, if using. Transfer mixture to prepared pan. Let cool completely, about 2 hours. Cover and refrigerate until set, about 2 hours.

Step 4

Using foil overhang, lift fudge out of pan. Cut into 1-inch cubes. Let sit at room temperature for 15 minutes before serving. (Fudge can be stored in airtight container at room temperature for up to 2 weeks.)