Preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper.
Place a small piece of parchment paper on to a small plate. Scoop out a 1 ½ tbsp. scoop of pistachio cream and place it on to the parchment paper. Place the pistachio cream in the freezer while you make the cookie dough.
Add the butter to a small saute pan or saucepan. Set the butter over medium heat, stirring occasionally until the butter bubbles up, then turns amber in color (this took me ~2 minutes.)
Pour the brown butter into a small mixing bowl and set it to chill in the freezer for 5 minutes.
Once the butter has chilled, mix in the light brown sugar and granulated sugar, followed by the milk and vanilla.
Add in the flour, baking soda, and salt. Mix together just until a few flour streaks remain, then mix in the chocolate chips and pistachios just until no flour streaks remain.
Remove the pistachio cream from the freezer. Scoop out all of the cookie dough from the mixing bowl and flatten it in your hand. Place the pistachio cream on top of the center of the cookie dough, then fold the cookie dough up into a ball around the pistachio cream.
Set the cookie in the freezer to chill for 5 minutes.
Top the cookie with extra pistachio cream, chocolate chips, and pistachios as desired. Bake for 15-17 minutes, until the edges are golden brown and the middle of the cookie no longer looks “wet.”
Allow the cookie to rest on the hot baking sheet for 5-10 minutes before transferring it to a wire rack to cool (or just before enjoying it warm.) Serve and enjoy!