Heat oven to 375ºF with rack set in middle position. Line two baking sheets with parchment paper.

Whisk together flour and baking soda; set aside.

Melt 8 tablespoons butter in a skillet over medium-high heat. Continue cooking melted butter, swirling pan constantly, until butter is dark golden brown and has a nutty aroma, 3–4 more minutes. Transfer browned butter to a large bowl, add remaining 2 tablespoons butter and stir until completely melted.

Add brown sugar, tahini, granulated sugar, vanilla, and salt and whisk until fully incorporated. Add egg and yolk and whisk until mixture is smooth and no lumps remain, 30 seconds. Let mixture stand for 3 minutes then whisk vigorously for 30 seconds. Repeat process of resting and whisking 2 more times until mixture is thick, fluffy, smooth and shiny.

Using a rubber spatula, stir in flour mixture until just combined, about 1 minute. Stir in chocolate chunks and sesame seeds, if using, just until incorporated.

Working with 3 tablespoons (#24 scoop) dough at a time, roll into balls and space 2 inches apart on prepared sheets. Bake 1 sheet at a time until cookies are golden brown, puffy, and edges have begun to set but centers are still soft, 10–12 minutes, rotating sheet halfway through baking. Transfer sheet to a wire rack and sprinkle cookies with sea salt; let cookies cool to room temperature.

Store cookies in an airtight container at room temperature for up to 5 days.