Line a baking sheet with parchment paper and set aside.
Melt the chocolate over a double boiler (or in 30-second intervals in the microwave on 50% power, stirring after each). Once the chocolate is melted, remove from the heat and let sit for a few minutes to cool slightly, stirring occasionally. Add the marshmallows and walnuts and stir to combine.
Spread the chocolate mixture onto the prepared pan in an even layer, covering about a 7x7-inch space. It doesn't have to be perfect, but that's a good guideline.
Refrigerate for at least 30 minutes, or until set.
Using a sharp knife, cut the bark into pieces. Store in an airtight container in the refrigerator.
Note: I recommend Ghiradelli or Lindt chocolate.Nutritional values are based on one serving