The night before making the cookies, bring two cups of sugar and one cup of water to a rapid boil. Do not stir. Once the sugar mixture has reached 300°F, quickly pour it onto a silicone mat in a thin, even layer. Let it cool overnight. Break the solid sugar into shards, then place it in a plastic bag. Use a meat mallet or the bottom of a pan to smash the sugar into small, pebble-sized pieces. Try not to over-crush; you want a mix of small pebble to powder in size.
Preheat the oven to 350°F. In a large bowl, beat together the butter, eggs, and vanilla extract until smooth. Stir in the flour, baking soda, salt, and cocoa powder until a thick and sticky dough forms. Use an ice cream scoop to measure out 6-8 balls of dough per baking sheet. Bake for 10-12 minutes.
Remove the cookies from the oven. Let the cookies rest for about five minutes, then transfer to rounded custard bowls or similar concave shapes. Gently press another bowl into the middle of the cookie to help evenly distribute the cookie while it finishes cooling. Let the hot cookie dough cool completely in the custard bowls. Use a knife along the edge to loosen the cookies from the bowls when cooled.
While the cookies cool, sort the Jolly Ranchers into individual colors and place inside plastic baggies. Use a meat mallet or pan to crush the candies into small shards. Prepare your favorite vanilla icing recipe or use premade vanilla icing—spoon into a piping bag.
Pipe the interiors of each cookie with a thin layer of icing. Press the crushed Jolly Ranchers in the center and the caramel sugar around the edges. Press until the candies are secured in the icing. Chill until ready to serve. Place color-coordinated halves on top of the cookies, as desired, for a "real" geode effect. Enjoy!