1. Freeze pistachio spread: Line a plate with parchment paper. Scoop pistachio spread into dollops and freeze for at least 1 hour.
2. Prep: Line a cutting board or plate with parchment paper. Set aside.
3. Flour mixture: In a bowl, combine flour, cornstarch, baking powder, baking soda, and salt.
4. Butter mixture: In a large bowl, whisk melted butter, brown sugar, and granulated sugar until combined. Add egg and vanilla bean paste; whisk until smooth.
5. Combine mixtures: Add flour mixture to butter mixture, whisking until just combined.
6. Fold in chocolate and pistachios: Use a spatula to incorporate chocolate and pistachios into the dough.
7. Scoop dough: Use a cookie scoop to form dough balls. Chill in fridge for about 1 hour.
8. Prep oven: Preheat oven to 350°F. Line a baking sheet with parchment paper.
9. Stuff dough: Flatten each dough ball, place pistachio cream in the center, and seal.
10. Toppings: Place on baking sheet, add chocolate and pistachios on top.
11. Bake: Bake for 11-12 minutes. Sprinkle with flaky salt. Cool on baking sheet for 5 minutes before serving.