Freeze pistachio spread: Line a plate with parchment paper. Pour the pistachio spread on top and spread it into a thick sheet. If your pistachio spread is thick enough, you can also scoop it out into small dollops with a spoon. Freeze for at least 1 hour or overnight until solid. If you've gone with the sheet method, use a knife to cut the frozen sheet into squares and pop it back in the freezer until needed.

Prep: Line a cutting board or plate with parchment paper. This will be used to chill the cookie dough on. Set aside.

Flour mixture: In a small bowl, stir together flour, cornstarch, baking powder, baking soda, and salt. Set aside.

Butter mixture: In a large mixing bowl, add the melted and cooled butter, brown sugar, and granulated sugar. Whisk until combined. Add the egg and vanilla extract. Whisk until smooth and thick.

Combine flour and butter mixtures: Add the flour mixture and whisk until just barely combined with some flour patches left.

Fold in chocolate and pistachios: Add the chopped chocolate and chopped pistachios. Switch to a spatula to fold the chocolate and pistachios in and finish mixing the dough until just combined.

Scoop cookie dough: Use a 4-tablespoon cookie scoop to scoop the cookie dough. Release the cookie dough balls onto your lined board. Chill the dough in the fridge for about 1 hour or until firm.

Prep: Preheat the oven to 350°F. Line a baking sheet with parchment paper.

Stuff cookie dough: Use your fingers to press a dough ball into a flat round. Place a frozen stack (or a dollop) of pistachio cream (about 3-4 squares depending on the thickness) in the middle and close the cookie dough around it to seal it in the middle.

Add toppings: Place the stuffed cookie dough balls on your lined baking sheet, leaving about 2" in between each cookie for spreading. Press a few more chocolate chunks, pistachios, and frozen pistachio cream chunks on top of each cookie dough ball.

Bake: Bake cookies for about 11-12 minutes or until the edges are set but the cookies are still soft. Immediately sprinkle with flaky salt. Let the cookies cool on the baking sheet for 5 minutes and then enjoy them warm for gooey centers or let them cool. The pistachio filling will firm up as they cool.