Preheat oven to 350 degrees. Line two baking sheets with parchment paper, set aside.
In a medium-sized bowl, whisk cocoa powder, 1 cup powdered sugar, cornstarch and salt together making sure there are no lumps. Set aside. In another bowl (microwave-safe), place 1 cup of chocolate chips and melt in microwave in 30 second increments until smooth. Set aside to cool slightly.
In the bowl of your stand mixer fitted with the whisk attachment, whip egg whites until stiff peaks form, about 3 minutes. Switch to paddle attachment and slowly mix in 1 cup of powdered sugar until smooth. Add cocoa mixture and mix until combined. Pour in warm melted chocolate and 1/2 cup chocolate chips and stir until completely combined. Dough should be very stiff and thick. If it's not, let it rest for a few minutes without touching it and it will stiffen up.
Place 1/2 cup powdered sugar in a small bowl. Roll 2 rounded tablespoons of dough into a ball and roll in powdered sugar as thin or thick as you want. Place on prepared baking sheet and repeat this process with remaining dough. Bake cookies for 9-10 minutes but no longer than 10 minutes or they will burn. They will look underdone, but that is how they are supposed to look. Allow them to cool on baking sheets for about 5 minutes or until able to be easily removed without breaking.
Gooey Dark Chocolate Truffle Cookies will stay fresh for up to 4 days. They will stiffen up on the second day, so if you want them gooey like they are when they are fresh out of the oven, pop them in the microwave for 10 seconds and they're good as new!