Spread 4 cups mini marshmallows onto a large baking sheet and freeze for 30 minutes.
In the meantime, break chocolate bars into smaller pieces and place in a large bowl. Add chocolate chips and butter pieces. Set aside.
In a large, heavy saucepan, combine sugar and evaporated milk. Heat over medium-high heat and stir with a whisk until sugar is dissolved.
Add 2 1/2 cups mini marshmallows and bring to a boil, stirring constantly with a wooden spoon. Continue to to cook at a light but steady boil for 7 minutes, stirring constantly.
Remove from heat and immediately pour over chocolate and butter.
With a wooden spoon, stir until chocolate and butter are completely melted and mixture is well blended.
Continue to stir until candy loses some of its shine.
Stir in frozen marshmallows, vanilla, and walnut halves.
Pour into a buttered 9x13 pan and refrigerate from at least 90 minutes before cutting. Keep refrigerated until ready to serve.