01 - Line a baking sheet with parchment paper or a silicone mat.
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.
03 - Remove the chocolate from the heat. Reserve 2 tablespoons of the melted chocolate in a small bowl.
04 - Sift the matcha powder into the remaining larger portion of melted chocolate and mix thoroughly until evenly colored.
05 - Pour the matcha chocolate onto the prepared baking sheet and spread into an even layer (about 1/4-inch thick) using an offset spatula.
06 - Drizzle the reserved plain white chocolate over the surface and use a toothpick or skewer to create a marbled effect.
07 - Immediately sprinkle toasted black sesame seeds and (if using) white sesame seeds evenly over the top. Finish with a pinch of flaky sea salt, if desired.
08 - Refrigerate for 30 minutes or until completely set.
09 - Break into pieces and store in an airtight container at cool room temperature for up to 1 week.