In a large bowl, using a stand mixer or a hand-held mixer, beat the butter, tahini, granulated sugar and brown sugar on medium speed until fluffy (about 2-3 minutes), scraping down the sides of the bowl as needed.
Add the egg, extra yolk and vanilla. Continue to mix for another minute, until the eggs are well-incorporated.
In a small bowl, whisk together the flour, baking soda and salt. Gradually add the flour mixture to the egg mixture, mixing on low speed until just combined. Gently stir in the chocolate chunks with a spatula, being careful not to overmix. Cover the dough and refrigerate for at least 2 hours or overnight.
Preheat oven to 350° (177°C). Line 2 cookie sheets with parchment paper or silicone baking mats. Roll cookie dough into 2 inch balls and place on cookie sheets, leaving about 3 inches between each cookie. Sprinkle the top of each ball with sesame seeds and gently press the seeds in.
Bake one sheet at a time, in the middle rack of the oven for 12-15 minutes. Once done, the cookies will be slightly brown around the edges, but pale in the center. They will continue to cook as they cool. Remove from oven, sprinkle with sea salt if using, and place cookie sheet to cool for 10 minutes. After 10 minutes, transfer the cookies to a wire cooling rack. Repeat with remaining cookies.