Line a baking sheet with parchment paper and set aside.

Melt the chopped chocolate using one of these methods: - Double boiler: place a heatproof bowl over a pot of simmering water (bowl should not touch the water). Stir the chocolate until smooth and fully melted. - Microwave: heat the chocolate in a microwave-safe bowl in 30-second intervals at 50% power, stirring between each interval, until mostly melted. Stir until smooth.

Remove the chocolate from the heat and let it cool for about 3–5 minutes, stirring occasionally, until it is slightly thickened but still pourable.

Add the miniature marshmallows and coarsely chopped walnuts to the warm chocolate and stir gently until the marshmallows and walnuts are evenly coated and distributed.

Pour the mixture onto the prepared parchment and use an offset spatula or the back of a spoon to spread it into an even layer about 7 x 7 inches (it does not need to be exact).

Refrigerate the baking sheet for at least 30 minutes, or until the chocolate is completely set and firm.

Remove the bark from the parchment and cut into pieces with a sharp knife, or break into pieces by hand. Store the pieces in an airtight container in the refrigerator.