Line a plate with parchment paper. Spread the pistachio cream into a thick sheet or scoop into dollops. Freeze for at least 1 hour or until solid. If using the sheet method, cut into squares and return to the freezer until ready to use.

In a small bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt. Set aside.

In a large bowl, whisk melted butter, brown sugar, and granulated sugar until combined. Add egg and vanilla extract. Whisk until smooth and slightly thickened.

Add the dry ingredients to the wet mixture. Mix until just combined, leaving a few flour streaks.

Using a spatula, gently fold in chopped chocolate and chopped pistachios until evenly distributed.

Scoop the dough into large balls (about 4 tablespoons each). Place on a parchment-lined board and refrigerate for 1 hour until firm.

Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.

Flatten each dough ball into a round disc. Place a frozen pistachio cream piece in the center and seal the dough around it completely.

Place stuffed dough balls on the baking sheet, leaving 2 inches between each cookie. Press extra chocolate chunks, pistachios, and pistachio cream pieces on top.

Bake for 11–12 minutes, until edges are set but centers remain soft. Immediately sprinkle with flaky salt.

Let cool on the baking sheet for 5 minutes before serving warm for a gooey center, or allow to cool completely for a firmer pistachio filling.