1. Melt the semi-sweet chocolate on the stove-top in a double boiler or in the microwave at 50% power, about 4 minutes, stirring frequently.
2. Line a baking sheet with parchment paper. Pour the melted chocolate onto the sheet and spread it out with a spatula. The chocolate does not have to be perfectly level; the change in thickness is a nice touch.
3. Sprinkle the melted chocolate with the marshmallows and graham crackers and allow to harden in a cool, dry place for about 45 minutes or until firm.
4. Melt the milk chocolate, cool slightly, and drizzle over the marshmallows and graham crackers. Break the bark into small pieces and store in an airtight container in the refrigerator for up to a week.