Begin by preparing the pistachio cream filling. Spread the pistachio cream onto a piece of parchment paper in a thin, even layer or create small dollops. Place it in the freezer for about 1-2 hours, until it is completely solid. Once frozen, cut the cream into squares if you used a sheet.

While the pistachio cream is freezing, line a cutting board or plate with parchment paper to prepare for your cookie dough.

In a large bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and salt until well mixed.

In another bowl, whisk the melted and cooled unsalted butter with the brown sugar and granulated sugar until smooth and creamy. Then, add the large egg and vanilla extract, and whisk again until well combined.

Gradually add the flour mixture into the butter mixture, stirring gently to ensure everything is just combined.

Fold in the roughly chopped chocolate and pistachios using a spatula.

Scoop out portions of the cookie dough onto your lined cutting board or plate, spacing them out for easy handling. Cover the dough and chill in the fridge for about 1 hour.

While your dough is chilling, preheat your oven to 350°F (175°C) and prepare a baking sheet by lining it with parchment paper.

Once the dough is chilled, take a portion of it and flatten it slightly in your palm. Place a square of the frozen pistachio cream in the center and carefully enclose the dough around it.

Place the stuffed cookies on the prepared baking sheet, leaving space between each cookie. Sprinkle some additional chopped chocolate and pistachios on top if desired.

Bake the cookies for about 12-15 minutes until the edges are set but the centers remain soft. Sprinkle with flaky salt as soon as they come out of the oven.

Allow them to cool on the baking sheet before transferring to a wire rack to cool completely.