Line a baking sheet or countertop with a sheet of waxed paper.
Put 12 ounces semi-sweet chocolate chips in a heatproof bowl. Melt the chocolate either: (a) in a double boiler over simmering (not boiling) water, stirring until smooth, or (b) in the microwave at 50% power in 20–30 second intervals, stirring between intervals, until smooth.
Let the melted chocolate sit 1–2 minutes so it is warm but not hot (this helps prevent the marshmallows from melting).
Add 4 cups miniature marshmallows and 1/3 cup chopped nuts to the melted chocolate. Fold gently with a spatula until the marshmallows and nuts are evenly coated.
Spoon the mixture onto the prepared waxed paper. Use the paper and a spatula or your hands to shape the mixture into a log.
Wrap the log completely in the waxed paper (twist the ends or cover snugly) and place it in the refrigerator. Chill until firm, about 2 hours.
Unwrap the chilled log and place it on a cutting board. Slice into 1/2-inch pieces with a sharp knife (wipe the knife between cuts if the chocolate sticks).