Start by lining a 9x13 inch baking dish with parchment paper.
In a double boiler or a microwave-safe bowl, combine the semi-sweet baking chocolate and butter. Heat until melted and smooth.
Remove from heat and stir in vanilla extract.
Add the quartered marshmallows and chopped walnuts to the melted chocolate mixture and gently fold until coated.
Pour the mixture into the prepared baking dish and spread evenly.
Chill in the refrigerator for at least 2 hours until firm.
Remove from the baking dish and cut into squares or rectangles to serve.
