Line a large baking sheet with parchment paper or a silicone mat and set aside.

Melt the chopped dark chocolate in a double boiler over simmering water, stirring frequently until smooth and completely melted. Remove from heat and stir in vanilla extract.

Pour the melted chocolate onto the prepared baking sheet and use an offset spatula to spread it into an even layer about 1/4-inch thick, forming a rough rectangle shape.

If your caramel sauce is thick, warm it in the microwave for 10-15 seconds until pourable. Drizzle the caramel over the chocolate in random lines across the surface.

Use a toothpick or skewer to swirl the caramel through the chocolate in a figure-eight pattern, creating marbled waves. Avoid over-swirling or the colors will muddy together.

Immediately sprinkle the flaky sea salt evenly over the surface, and add optional toppings like crushed pretzels or nuts if using.

Refrigerate the bark for 15-20 minutes until completely set and firm to the touch.

Remove from the refrigerator and let sit at room temperature for 5 minutes. Break the bark into irregular pieces by hand or cut into squares with a sharp knife. Serve immediately or store in an airtight container.