Yield: makes 18 cookies
Preheat oven to 350 degrees F. Line baking sheets with parchment paper or silicone baking mat. Set aside.
In a large bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream together butter, brown sugar, and confectioners' sugar. Scrape down bowl as needed to ensure thorough mixing. Beat for about 3 minutes until mixture is smooth and creamy.
Add egg, egg yolk, and vanilla. Mix until incorporated.
With the mixer running on slow, add flour mixture in three additions. Mix until there are no longer any dry streaks of flour. Scrape down bowl as needed. Fold in walnuts and halva chunks until distributed.
Use a size 24 cookie scoop to portion out cookie dough. Place cookie mounds on prepared baking sheet, spacing them about 2-inches apart.
Bake for 12-15 minutes, rotating baking sheet halfway through baking, until edges of cookies are set. Immediately sprinkle sea salt flakes over cookies. Cookies will be VERY soft directly from the oven. Allow cookies to cool on baking sheet for 8-10 minutes before transferring to wire rack.
Cookies will be very fragile while hot, but will regain their structure as it cools.
Store cooled cookies in an airtight container for up to 5 days. They will remain soft and chewy for at least 4 days. To keep cookies super soft, add a slice of sandwich bread or use a terracotta sugar keeper.