Preheat oven to 350°F (180°C).

In a large bowl, stir together brown sugar, tahini, melted butter, and vanilla with a wooden spoon until smooth. Add eggs, and stir until combined.

In a medium bowl, whisk together flour, salt, and baking soda. Add flour mixture to sugar mixture, stirring just until combined. Stir in chopped chocolate. Refrigerate for at least 1 hour or overnight.

Using a 1-ounce ice cream scoop or heaping tablespoon, scoop dough, and roll into smooth balls. Place 12 balls each on 2 ungreased baking sheets. Lightly press each ball with the palm of your hand, and sprinkle with a pinch of sesame seeds, if desired.

Bake until set and lightly browned at the edges, 10 to 15 minutes, rotating pans front to back and top to bottom halfway through baking. Let cool on pans for 1 minute. Remove from pans, and let cool completely on wire racks. Repeat with remaining dough. Store in an airtight container for up to 5 days.