In a large bowl, mix melted butter and both sugars together until combined. Stir in the egg, egg yolk and vanilla until smooth. Add flour, corn starch, salt and baking soda and mix until just combined. Fold in chopped pistachios and dark chocolate reserving 2 tablespoons of each to press into dough before baking (optional). Do not over-mix or this can result in tough cookies. Cover bowl with plastic wrap and chill dough for 30 minutes - one hour.
Preheat oven to 325 degrees. Line two cookie sheets with parchment paper or grease lightly. Shape your dough into one rounded tablespoon per each cookie and place on cookie sheet 1 1/2 inches apart. Take your reserved dark chocolate and pistachios and press them into the outside of each dough ball all over (again, optional). Bake for 10-11 minutes. Cookies will appear under-done, this is what you want if you want soft-baked cookies, they will set up just fine when they cool. If you like crispy cookies, bake them longer.
Allow cookies to cool on hot pan for no longer than 5 minutes, then transfer to a wire rack carefully to finish cooling completely.
Pistachio Dark Chocolate Chunk Cookies will stay soft and fresh for up to 4 days stored in an airtight container at room temperature.