Line a large baking sheet with parchment paper or a silicone mat. Set aside in a cool, dry spot while you prepare the chocolate.

Melt the chopped chocolate in a heatproof bowl set over a pot of simmering water, stirring occasionally until smooth and glossy. Alternatively, microwave in 20-second intervals, stirring between each, until fully melted. Remove from heat and let cool for 2-3 minutes until slightly thickened.

Pour the melted chocolate onto the prepared baking sheet and spread into an even layer about 1/4-inch thick using an offset spatula. Work quickly before the chocolate begins to set.

Immediately press the graham cracker pieces into the chocolate in a single layer, spacing them evenly across the surface. Press gently so they adhere but don't sink completely.

Scatter the mini marshmallows over the chocolate, pressing them lightly into the surface. For extra flavor, you can toast the marshmallows under a broiler for 1-2 minutes before adding them.

Sprinkle with flaky sea salt if desired, then refrigerate for 15-20 minutes until the chocolate is completely firm and set.

Once hardened, lift the bark off the parchment using both hands and break into irregular pieces by pressing down firmly with a large knife. Store in an airtight container at room temperature.