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 until smooth. Alternatively, microwave in 20-second bursts, stirring between each, until fully melted and glossy.
03 - Remove the chocolate from the heat and reserve 2 tablespoons of the melted chocolate in a small separate bowl for the marble effect.
04 - Sift the matcha powder into the remaining larger portion of melted chocolate and mix thoroughly until the color is evenly distributed and no lumps remain.
05 - Pour the matcha-infused chocolate onto the prepared baking sheet and spread into an even layer approximately 1/4-inch thick using an offset spatula.
06 - Drizzle the reserved plain white chocolate over the surface in a random pattern, then use a toothpick or skewer to gently swirl and create a marbled effect.
07 - Immediately sprinkle toasted black sesame seeds and white sesame seeds (if using) evenly over the top while the chocolate is still soft. Finish with a pinch of flaky sea salt if desired.
08 - Refrigerate for 30 minutes or until completely set and firm to the touch.
09 - Break the bark into irregular pieces and store in an airtight container at cool room temperature for up to 1 week.