4 tablespoons (1/2 stick) unsalted butter, plus more for pan
3 cups mini marshmallows
1 cup sugar
1/2 cup heavy cream
1/2 teaspoon salt
1/2 teaspoon pure vanilla extract
1 bag (12 ounces) semisweet chocolate chips
1 cup walnuts, coarsely chopped
Butter an 8-inch square baking pan; line with 2 crisscrossed pieces of parchment paper, leaving an overhang. Butter paper.
In a large saucepan, combine butter, marshmallows, sugar, cream, and salt; cook over medium, stirring occasionally, until marshmallows are melted, 8 to 10 minutes. Off heat, stir in vanilla, chocolate chips, and 1/2 cup walnuts until chocolate is melted.
Pour fudge into prepared pan, and smooth top; sprinkle with remaining walnuts. Let cool 30 minutes. Refrigerate until set, about 1 hour.
Using overhang, lift fudge onto a cutting board; cut into squares (6 rows by 6 rows).