In the bowl of a stand mixer, add the butter, tahini, granulated sugar, and brown sugar. Beat on medium speed for 2-3 minutes or until fluffy.
Using a spatula, scrape down the sides of the mixer. Ad the egg, and beat for 1 minute. Add the egg yolk and vanilla extract, and beat again until the ingredients are combined and fluffy.
In a separate bowl, whisk the flour, oats, walnuts, baking soda, and sea salt together. With the mixer on low speed, add the dry ingredients to the bowl, mixing just until combined. Gently stir in the chocolate chips. Do not overmix!
Using a large cookie dough scoop, portion the dough into 1 ½ inch balls. Cover the dough, and refrigerate it overnight. Or, place it in the freezer for 1 hour.
Preheat the oven to 350 degrees Fahrenheit. Grease a cookie sheet.
Arrange the cookie dough balls in an even layer, leaving 3 inches between each ball of dough.
Bake 13-14 minutes or until the edges are just turning golden brown.
Remove from the oven, sprinkle the Maldon sea salt on top, and let the cookies cool slightly on a baking sheet before transferring them to a wire rack to cool completely.
Store in an airtight container at room temperature for up to 5 days.
