Step 1Lightly butter an 8-inch-square baking pan and set aside.
Step 2Combine the butter, chocolate and vanilla in a bowl. Set aside.
Step 3Place the sugar, evaporated milk and marshmallows in a saucepan. Bring to a boil over medium heat, stirring frequently. Reduce the heat to low and cook, stirring constantly, until the marshmallows have melted and the mixture has thickened a bit, about 6 minutes.
Step 4Pour the hot mixture over the ingredients in the bowl. Beat with an electric mixer until the fudge is thick and dull, 4 to 5 minutes. Stir in the nuts.
Step 5Pour into the pan. Refrigerate several hours to firm, then cut into 24 pieces.