Use a damp towel to wipe your silicone mold clean. Use a dry towel to dry the cavities out completely. Place the silicone mold on a medium sheet pan and set aside.
Add ¾ cup chocolate chunks and the coconut oil to a medium bowl. Microwave for 30 seconds and then stir. Microwave 30 seconds more and stir. The chocolate should be completely melted after you have stirred it well. If there are still some chunks visible, microwave in 10 second increments, removing to thoroughly stir, until melted completely.
Use a small brush (I used silicone) to brush each cavity with the melted chocolate. The chocolate should cover each cavity so that no "holes" are visible. Place in the refrigerator for 30 minutes to chill. Remove from refrigerator and repeat once more until there is a thick chocolate coating all over the inside of each cavity. Return the silicone mold to the refrigerator to chill for another 15 minutes. Place any leftover chocolate mixture (you should have a few tablespoons' worth) into a small saucepan.
To assemble the sundaes, place two chocolate chip cookies in the center of a small bowl. Carefully remove one chocolate shell from its mold and place it over the cookies. Let sit out at room temperature for 15-20 minutes so that the chocolate shell starts to warm up a bit.
When ready to serve, add 3 tablespoons of milk to the saucepan with the chocolate. Turn the burner on to low and stir until the chocolate and milk are combined and warmed through. Remove from heat.
Add any other sundae topping you would like (ice cream, fresh fruit, chopped nuts, fresh mint, etc.) around the chocolate bomb, leaving the top and sides of the chocolate bomb untouched.
When ready to serve, spoon the warm chocolate sauce mixture over the top of the chocolate shell and watch as the chocolate gives way to the cookies hiding underneath! Serve immediately and enjoy!
