Form about 8 small balls (1 Tbsp each) of pistachio cream and freeze for at least 1 hour until solid.

Melt and brown the butter in a saucepan until golden and nutty-smelling. Let it cool slightly, then whisk in both sugars.

Whisk in the egg and vanilla until smooth. Add dry ingredients—flour, baking powder, baking soda and salt—and fold until just combined.

Fold in chopped chocolate and pistachios until no streaks of flour remain. Chill dough for about 30 minutes or longer to firm up.

Preheat oven to 350 °F (175 °C). Scoop dough into eight large balls (3–4 Tbsp each). Flatten one ball in your palm, press a frozen pistachio cream ball into the center, then completely enclose it in dough. Roll into a smooth ball.

Place stuffed cookies about 2″ apart on parchment-lined baking sheets. Optionally press a few extra chocolate chunks or pistachios on top and drizzle a little extra pistachio cream.

Bake 12–14 minutes until edges are golden but centers still soft. Immediately sprinkle with flaky salt if desired. Let cookies rest on the baking sheet for 5 minutes before transferring to a rack.