Whisk together 1¼ cups (160g) flour and ¾ teaspoon 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 bowl of a stand mixer fitted with the whisk attachment, add remaining 2 tablespoons butter and stir until completely melted.
Add 1 cup (200g) brown sugar, 1 teaspoon vanilla and 1 teaspoon salt; beat until fully incorporated.
Add 1 large egg and 1 large egg yolk and beat vigorously until mixture is light pale in color, fluffy, 3 minutes.
Add ½ cup (130g) black tahini and mix until combined.
Add flour mixture and mix for 1 minute on medium speed.
Cover dough with plastic wrap and refrigerate 1 hour.
Divide dough into 14 portions, each about 3 tablespoons (or use #24 cookie scoop) and arrange on a baking sheet.
Heat oven to 350ºF (177ºC) ; line baking sheets with parchment paper.
Arrange 6 dough balls, 3-inches apart, on a prepared sheet. (Smaller baking sheets can be used, but will require multiple batches. Bake 3 cookies at a time on quarter sheet pans).
Transfer sheet to oven and bake until edges are set but centers still look puffy and raw, about 14 minutes, rotating pan halfway through baking. Transfer sheet to a wire rack and cool on sheet 3 minutes then transfer cooking to a wire rack to cool completely.
If using chocolate, melt chocolate in a microwave-safe bowl in microwave in 30-second increments on 50% power until melted, stirring after each heating.
Drizzle cookies with chocolate and top with sprinkles, if using. Allow chocolate to set.