Line a 9x13-inch pan with aluminum foil. Coat the foil with buter. Put 4 cups of the marshmallows on a baking sheet in a single layer and stick it into the freezer so they can get cold.

In a large pot, combine the remaining two cups of marshmallows (or 20 large marshmallows), sugar and evaporated milk over medium-high heat. Stir together as the marshmallows melt and the mixture comes to a boil. Set a timer and boil for 6 minutes, stirring constantly. Don't be alarmed by brown bits that might appear while cooking as long as your temperature isn't too high and you are stirring consistently.

When the timer goes off, 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, then let the fudge sit for just 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 from the residual heat of the fudge. Pour into the prepared pan lined with butter-coated aluminum foil 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.