Line an 8-inch square metal pan with foil and grease lightly with butter. In a large heatproof bowl, combine the chocolate chips, marshmallows, vanilla, and nuts. Set aside.

In a heavy 3-quart saucepan, combine the sugar, evaporated milk, butter, and salt. Cook over medium heat, stirring constantly, until the mixture comes to a full rolling boil.

Once boiling, set a timer for 4 minutes. Keep at a gentle boil, stirring constantly. Remove from heat when the timer goes off.

Pour the hot mixture over the chocolate bowl in three additions, stirring well after each. Stir until completely smooth and glossy.

Pour into the prepared pan and smooth the top. Let sit at room temperature for 1 hour, then refrigerate for at least 4 hours or overnight.

Lift out using the foil, peel away foil, and cut into 1-inch squares. Store in a sealed container in the fridge for up to 10 days.