Line a 9x9 square baking pan with wax paper, leaving a 2 inch overhang on opposites sides.

Mix the marshmallows and walnuts in a large mixing bowl and set aside.

Combine the condensed milk, butter, and chocolate chips in a large saucepan.

Cook over medium heat, stirring frequently, until melted and smooth.

Let cool 5 minutes.

Pour the chocolate mixture over the marshmallow mixture and mix until thoroughly combined.

Pour the mixture into the prepared pan and smooth the top with a buttered spatula.

Put the pan in the refrigerator for 2 hours or until set.

Using the wax paper overhang, lift out of pan and place on a cutting board.

With a sharp knife, cut into bars, cleaning the knife blade in between cuts.