Preheat oven to 350F. Line 1-2 baking pans with parchment paper.
Brown the butter by adding the cubed butter to a saucepan over medium heat. Once melted, stir consistently until brown specks form at the bottom and it smells nutty, about 8-10 minutes. Let cool 10 minutes.
Add the sugars and browned butter to the bowl of a stand mixer. Beat for 2-3 minutes.
Add in the egg, egg yolk, milk, and vanilla. Beat for 1 minute.
Add the flour, baking soda, salt, and baking powder. Mix just until combined. Be careful not to over mix the batter!
Fold in half of the chopped chocolate, reserving the other half for topping later.
Using 2-3 tbsp cookie dough per cookie, flatten in the palm of your hands, spoon ½ tbsp to 1 tbsp pistachio cream in the center depending on how large you want the cookie, and fold over the cookie dough ends to seal it together. Roll into a ball, then slightly smash to about ¾" thick.
Top each cookie ball with the reserved chopped chocolate.
Bake for 10-12 minutes, until the edges are lightly golden brown. Use a large circular cookie cutter to help reshape the cookies after baking, if needed.
Let cool on the pan 5 minutes before transferring to a wire rack to cool completely... or enjoying warm!
(Optional) Garnish with small spreads of more pistachio cream and flaky salt.
