Combine the butter, chocolate, and vanilla extract in a medium bowl. Set aside.
Place the sugar, evaporated milk, and marshmallows in medium saucepan.
Bring to a boil over medium heat, stirring frequently. Monitor the temperature using a candy thermometer.
When the mixture reaches the soft-ball stage (234–240°F), reduce the heat to low and cook for 6 minutes, and do not stir the fudge during this cooking time.
Pour the hot mixture into the bowl with the butter, chocolate, and vanilla.
Beat with an electric mixer until the fudge is thick and dull (this doesn't take long).
Stir in the nuts.
Pour the fudge into a lightly buttered 8x8-inch baking pan.