In a medium saucepan, melt the butter over medium heat. Stir occasionally and cook until the butter foams and turns golden brown, releasing a nutty aroma. Remove the brown butter from the pan and transfer to a large mixing bowl. Let it cool to room temperature.*

While the brown butter is cooling, prepare the coating. Finely chop the pistachios, then add to a bowl with the cinnamon and sugar. Stir to combine. Preheat the oven to 350 degrees, and line a sheet pan with parchment paper.

Once the brown butter has cooled, add the granulated sugar and brown sugar. Cream the butter and sugars together until creamy and lighter in color, about 2 minutes. Then whisk in the vanilla, egg, and egg yolk.

In a separate medium bowl, whisk together the all-purpose flour, cream of tartar, cinnamon, baking soda, and salt.

Add the dry ingredients into the wet ingredients and gently fold until just a few flour streaks remain. Then, fold in the chopped pistachios until incorporated.

Scoop the cookies into 18 dough balls or 6-7 massive cookies**(see note).

Roll the cookie dough balls in the cinnamon sugar coating until fully covered. Place the coated dough balls on the sheet pan about 2 inches apart (or 3 cookies/sheet pan if making massive cookies), and repeat with the remainder of the cookie dough. Bake for 11-14 minutes (for smaller cookies) or 22-25 minutes (for massive cookies). The edges should be set, but the center still gooey.

Let cool, then enjoy! Sprinkle with any leftover cinnamon sugar coating and flaky salt.