Line a large baking sheet with parchment paper or a silicone mat. Set aside.
Melt the chopped dark chocolate in a heatproof bowl set over a pot of simmering water, stirring occasionally until smooth and completely melted. Remove from heat.
Pour the melted chocolate onto the prepared baking sheet and use an offset spatula to spread it into a thin, even rectangle about 1/4-inch thick.
In a small bowl, whisk together the tahini, maple syrup, and vanilla extract until smooth. Warm in the microwave for 10 seconds if the mixture is too thick to drizzle.
Drizzle the tahini mixture over the chocolate in lines or dollops across the surface.
Use a toothpick, skewer, or the tip of a knife to gently swirl the tahini into the chocolate, making 3-4 passes in different directions to create a marble pattern. Avoid over-mixing.
Sprinkle flaky sea salt evenly over the surface while the chocolate is still wet.
Refrigerate the bark for at least 2 hours until completely set and firm to the touch.
Once set, lift the bark off the parchment and break it into irregular pieces by hand. Store in an airtight container in the refrigerator.
