01 - Line a baking sheet with parchment paper or a silicone mat to prevent sticking and ensure easy removal.
02 - Melt the white chocolate in a heatproof bowl set over a pot of gently simmering water (double boiler method), stirring constantly until smooth and fully melted. Alternatively, microwave in 20-second bursts, stirring between each interval, until completely liquid.
03 - Remove the chocolate from heat source. Reserve 2 tablespoons of the melted white chocolate in a separate small bowl for marbling later.
04 - Sift the matcha powder into the remaining larger portion of melted chocolate, whisking thoroughly until the mixture achieves a uniform vibrant green color throughout.
05 - Pour the matcha-infused chocolate onto the prepared baking sheet, using an offset spatula to spread it into an even layer approximately 1/4-inch thick across the entire surface.
06 - Drizzle the reserved plain white chocolate over the matcha layer in random patterns, then use a toothpick or skewer to gently swirl through both chocolates, creating a decorative marbled effect.
07 - Immediately sprinkle the toasted black sesame seeds evenly over the surface, adding white sesame seeds if using for contrast. Finish with a pinch of flaky sea salt if desired, working quickly before chocolate begins to set.
08 - Transfer the baking sheet to the refrigerator and chill for 30 minutes or until the bark is completely firm and set throughout.
09 - Break the chilled bark into irregular pieces of desired size. Store in an airtight container at cool room temperature for up to 1 week, or refrigerate for extended freshness.