How to Make It
Lightly grease a 9-by-13-inch baking pan and line with parchment paper, leaving a 1-inch overhang. In a medium bowl, microwave 1 1/2 cups of the bittersweet chocolate chips in 30-second bursts until just melted; stir until smooth. Stir in the remaining 1/2 cup of bittersweet chocolate until melted. Spread in an even layer in the prepared baking pan.
In a small bowl, whisk the matcha powder with the 1 1/2 teaspoons of oil. In a large bowl, microwave 3 1/2 cups of the white chocolate chips in 30-second bursts until just melted; stir until smooth. Stir in the remaining ½ cup of white chocolate until melted.
Mix 1/2 cup of the melted white chocolate with the matcha until no streaks remain. Scatter large spoonfuls of the matcha chocolate and the white chocolate over the dark chocolate layer and, working quickly, use a small spatula to decoratively swirl the matcha chocolate with the white chocolate. Sprinkle with the sesame seeds. Let stand at room temperature until firm, 2 hours. Remove the bark from the pan, cut into pieces and serve.