Preheat the oven to 350*F.
Pop the popcorn by heating the oil in a saucepan with a tight fitting lid. When the oil is shimmering, add the popcorn kernels and toss gently to coat. Keep the lid on tight and heat until you hear the kernels begin to pop. Shake the pan gently while the kernels pop until there are no more popping sounds, then immediately remove the popcorn from the heat and pour into a bowl. Set aside to cool a bit.
Combine the flour, baking soda, salt and cocoa powder and whisk gently with a fork in a large bowl. Whisk the eggs and add the vanilla, butter and sugar. Slowly fold the wet ingredients in with the dry and combine until just mixed together.
Roll the dough into 2" wide balls and evenly distribute on a baking sheet lined with parchment paper. Press a well into each cookie with your thumb (the way you would if making thumbprint cookies). Gently press a small handful of popcorn onto the top of each cookie. It's okay if the edges of the cookie batter crack a bit during this.
Bake the cookies for 10-12 minutes or until the edges has lightly crisped. Let cool before eating.
Cookies will keep for three-four days if covered tightly and stored in a cool, dry place.