Line a large baking sheet with parchment paper or a silicone mat. Set aside in a cool area away from heat sources.
Melt the dark chocolate in a microwave-safe bowl in 30-second intervals, stirring between each, until completely smooth and glossy. Stir in 1.5 tablespoons espresso powder and vanilla extract until fully dissolved.
Pour the dark chocolate mixture onto the prepared baking sheet and spread into a thin, even rectangle about 10x12 inches using an offset spatula. Work quickly before it begins to set.
In a separate bowl, melt the white chocolate using the same method. Stir in the remaining 0.5 tablespoon espresso powder until you have a light mocha color.
Drizzle the white chocolate mixture over the dark chocolate in random lines. Use a toothpick or skewer to gently swirl the two chocolates together, creating a marbled pattern with 3-4 long strokes.
While the chocolate is still wet, immediately sprinkle the whole espresso beans evenly across the surface, pressing them lightly to help them adhere. Finish with a light sprinkle of flaky sea salt.
Transfer the baking sheet to the freezer for 15-20 minutes until the bark is completely firm and set. It should feel solid and snap cleanly when bent.
Remove from the freezer and break the bark into irregular shards by hand or cut into clean squares with a sharp knife. Store immediately in an airtight container to prevent bloom.
