Place miniature marshmallows on a cookie sheet and freeze.  You want to give them a couple of hours in the freezer.  This will prevent them from melting in the fudge.

In a large pot, combine the  sugar, milk and the 20 large marshmallows.  Bring to a boil and  boil for 6 minutes, stirring constantly. (Don't be alarmed by the brown bits that will appear while cooking the mixture.)

Remove from heat and mix in the chocolate chips and butter.  Stir until thick then add the walnuts.

Let it sit a little longer to thicken more and mix in the vanilla and frozen marshmallows.  Pour into greased 9 x 13 pan and allow to set.