This recipe is courtesy of Hershey's.

1. Line a square 20 to 22-cm (8 to 9-in.) baking pan with foil. 
2. Melt chocolate chips and 30 mL (2 tbsp.) butter with milk, vanilla and salt in a double boiler set over low heat. Remove from heat and stir in nuts. Spread in prepared pan. 
3. Melt marshmallows with remaining butter in a medium saucepan set over low heat. Spoon onto warm fudge. With a table knife or metal spatula, swirl through fudge. 
4. Chill for 2 hours or until firm. Remove fudge from pan, peel off foil, and cut into squares. Store at room temperature. 

Chef's secrets

For best results, do not double this recipe.
You can also bake this recipe in a mini muffin tin lined with non-stick parchment mini baking cups.