Begin by whisking the flour, cornstarch, baking soda, and salt together in a large bowl. Set this dry mixture aside while you prepare the rest of the dough.

Place ½ cup of the pistachios in a food processor and pulse several times until finely ground, but still retaining a little texture. Transfer them to a small bowl. Add the remaining ½ cup of pistachios to the processor and pulse until coarsely chopped. Set those aside as well.

In another bowl, combine the melted butter with both the brown sugar and granulated sugar. Whisk firmly until the mixture is smooth and there are no visible lumps of sugar. Add the egg, egg yolk, vanilla extract, and finely ground pistachios, then whisk until evenly combined.

Pour the butter mixture into the bowl of dry ingredients. Using a rubber spatula, gently mix until a soft dough forms. Fold through the chocolate chunks and chopped pistachios, making sure they are distributed throughout the dough.

Cover the bowl and place the dough in the fridge for at least 2 hours. Before shaping, take it out and let it sit at room temperature for 15 minutes, just until it becomes easier to work with.

Heat the oven to 160°C, 325°F, and line two large baking sheets with parchment paper. Divide and roll the dough into balls, shaping them slightly taller than they are wide. Arrange them on the prepared sheets, leaving plenty of room between each cookie for spreading.

Bake the cookies in batches for approximately 12 minutes, or until the edges look set while the centers remain slightly soft and jiggly. Take them from the oven and immediately finish with a light sprinkle of sea salt.

Let the cookies rest on the baking sheet for 10 minutes before transferring them to a wire rack. They are especially delicious served slightly warm, while the centers are still soft and gooey.

Note: Do not be concerned if the cookies appear a little under-baked when they first come out of the oven. They will continue cooking from the residual heat of the baking sheet. The 10-minute resting time is important, so allow them to set before moving them to the cooling rack.