In a sauce pan over the stove top on medium-low heat, melt chocolate with butter, constantly stirring until smooth. Allow mixture to cool for a few minutes, then whisk in powdered sugar. Mixture will become slightly pasty. Add whipped topping and walnuts and quickly stir until smooth and chocolate is fully incorporated. Pour chocolate over marshmallows and stir until fully coated.

Sprinkle a little graham cracker crumbs on a large piece of saran or cling wrap. Add half of marshmallow mixture over the crumbs and form it into a log shape with your hands. Sprinkle with additional graham crackers and roll up into wrap until fully covered. Do the same for the remainder of the marshmallow mixture, forming it into a log, then placing both in the refrigerator for at least one hour, or until the logs are set, stay together well, and are easy to slice.