Part 1: Prepare the Dough

1. Whisk Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. This ensures the leavening and salt are evenly distributed. Set aside.

2. Cream Butter and Sugars: In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat the room temperature butter, brown sugar, and granulated sugar on medium-high speed for 2-3 minutes, until the mixture is light, fluffy, and pale in color.

3. Add Wet Ingredients: Add the black sesame paste and beat on medium speed until it is fully incorporated and the mixture is a uniform grey color. Scrape down the sides of the bowl. Add the eggs one at a time, beating well after each addition. Beat in the vanilla extract.

4. Combine Wet and Dry: Turn the mixer to low speed and gradually add the flour mixture in three additions, mixing only until the last streaks of flour have disappeared. Be careful not to overmix, which can lead to tough cookies.

5. Fold in Chocolate: Remove the bowl from the mixer. Add the chopped dark chocolate and use a rubber spatula to fold it into the dough until it is evenly distributed.

Part 2: The Crucial Chilling Step

1. Chill the Dough: Cover the bowl with plastic wrap and refrigerate the cookie dough for at least 2 hours, and up to 48 hours. This step is mandatory. Chilling the dough solidifies the fat (preventing the cookies from spreading too much), allows the flour to hydrate fully (leading to a chewier texture), and lets the flavors meld and deepen.

Part 3: Bake the Cookies

1. Preheat and Prepare: When you’re ready to bake, preheat your oven to 375°F (190°C). Line two large baking sheets with parchment paper.

2. Scoop the Dough: Using a medium cookie scoop (about 2 tablespoons), portion the chilled dough into balls and place them about 2-3 inches apart on the prepared baking sheets.

3. Garnish: Lightly press a few extra chocolate chunks onto the tops of the dough balls. Sprinkle each one with a pinch of the mixed sesame seeds and a generous pinch of flaky sea salt.

4. Bake: Bake for 11-13 minutes, or until the edges are golden brown and set, but the centers still look slightly soft and underdone.

5. Cool: Let the cookies cool on the baking sheet for 5-10 minutes. This allows them to set up before you try to move them. Then, transfer the cookies to a wire rack to cool completely.