Line a 9 x 13-inch pan with aluminum foil. Coat the foil with butter. Put 4 cups of the mini marshmallows on a baking sheet in a single layer and place into the freezer so they can get cold.
In a large saucepan, combine the remaining two cups of mini marshmallows, sugar and evaporated milk over medium-high heat, stirring constantly as the marshmallows melt and the mixture comes to a boil. Boil for 6 minutes, stirring constantly.
After 6 minutes remove the fudge from the heat and immediately add the cold cubed butter and chocolate chips, stirring until completely combined.
Stir in the walnuts and vanilla, let the fudge sit for a few minutes to thicken and cool slightly before stirring in the frozen miniature marshmallows. You don't want to stir the marshmallows too much or they will melt. Pour into the prepared pan and transfer to the fridge to set for 3 to 4 hours.
Carefully lift the fudge out of the pan and peel off the foil. Cut the fudge into small squares to serve.
Notes
I like to store fudge wrapped in plastic wrap, then waxed paper, stored in a plastic baggie and kept in the refrigerator. This is how I store all the fudge I make:)