How to Make It
Line 2 baking sheets with parchment paper. Beat butter, sugar, and tahini in a medium bowl with a hand mixer on medium-high speed until light and fluffy, 3 to 5 minutes. Add egg, lemon zest and juice, and vanilla; beat until combined, about 1 minute, scraping down sides of bowl as needed. With mixer running on low speed, gradually add flour and salt; beat until dough comes together. If using, fold in cacao nibs until combined.
Divide dough in half; place each half on a large piece of plastic wrap. Fold plastic wrap over to cover dough, and, using your hands, roll each into a smooth log 1 inch in diameter. Refrigerate until firm, at least 1 hour and 30 minutes or up to 8 hours.
Preheat oven to 350°F. Combine sesame seeds on a small sheet pan or flat plate. Remove dough logs from plastic wrap, and roll in sesame seeds, pressing firmly so seeds adhere evenly. Slice dough logs into 1/4-inch-thick rounds. Place 1 inch apart on prepared baking sheets.
Bake cookies in preheated oven until golden around edges, 8 to 10 minutes. Let cool on baking sheets 5 minutes; transfer to a wire rack, and let cool completely, about 15 minutes. Store in an airtight container at room temperature up to 5 days.