Preheat your oven to 350°F (175°C) and prepare an 8x8 inch baking pan with butter and parchment paper.

Toast the black sesame seeds in a dry skillet for 20-30 seconds until fragrant, then grind into a fine powder.

Melt the unsalted butter in a saucepan, then stir in white chocolate until smooth.

In a mixing bowl, combine granulated and brown sugars, eggs, vanilla extract, and salt. Beat until thick and creamy.

Gradually pour in the chocolate-butter mixture while mixing on low speed until well combined.

Sift and fold in flour and ground sesame seeds until no flour streaks remain.

Pour batter into the prepared pan and bake for 20 minutes until edges are set and the center is soft.

Remove from oven, gently slam on counter to release air, sprinkle with flaky sea salt, and bake for an additional 15-20 minutes.

Let cool for at least 10 minutes before lifting out, then cut into squares and serve.