Toast the black sesame seeds in a dry skillet over medium heat for 2 to 3 minutes, stirring frequently, until fragrant. Be careful not to burn them. Transfer to a plate to cool, then grind half of them in a spice grinder or mortar and pestle until they become a fine powder. Leave the other half whole.
In a medium bowl, whisk together the almond flour, tapioca starch, ground black sesame powder, baking soda, and salt. Set aside.
In a large bowl, beat the softened butter, brown sugar, and granulated sugar together until creamy and fluffy, about 3 minutes. Add the egg, vanilla extract, and black sesame paste and beat until smooth and well combined.
Gradually add the dry ingredients to the wet mixture and stir until just combined. Fold in the white chocolate chunks and the remaining whole toasted black sesame seeds. Cover the dough and refrigerate for 1 hour.
Preheat your oven to 350F (175C) and line two baking sheets with parchment paper. Scoop the dough into 2 tablespoon-sized balls and place them 3 inches apart on the prepared sheets.
Bake for 11 to 13 minutes until the edges are golden and set but the centers still look slightly underdone. Remove from the oven and immediately press a few extra white chocolate chunks on top if desired. Sprinkle each cookie with a pinch of flaky sea salt.
Let the cookies cool on the baking sheets for 10 minutes before transferring to a wire rack. The cookies will be dramatically dark with pockets of melted white chocolate and a deeply nutty, toasty sesame flavor.