01 - Line a baking sheet with parchment paper or silicone mat.
02 - Melt white chocolate in a heatproof bowl set over a pot of gently simmering water, stirring until smooth. Alternatively, microwave in 20-second bursts, stirring between each, until fully melted.
03 - Remove chocolate from heat and reserve 2 tablespoons of melted chocolate in a small bowl for later marbling.
04 - Sift matcha powder into the remaining melted chocolate and mix thoroughly until evenly colored throughout.
05 - Pour matcha chocolate onto prepared baking sheet and spread into an even layer about 1/4-inch thick using an offset spatula. Drizzle reserved plain white chocolate over surface and use a toothpick to create marbled effect.
06 - Immediately sprinkle toasted black and white sesame seeds evenly over the surface. Finish with a pinch of flaky sea salt if desired.
07 - Refrigerate for 30 minutes or until completely set. Break into pieces and store in an airtight container at cool room temperature for up to 1 week.