Place skull mold on a small cutting board - this will make it easy to balance as you place it in the fridge. Bloom (dissolve) 1½ tsp. Knox Gelatin in 1 tbsp. cool water in a small bowl for 30 seconds. Stirring gently.
Heat ¼ cup white chocolate liqueur in a small saucepan for no more than 1 minute over low/medium heat. As soon as you see any boiling bubbles, turn the heat off. Stir in bloomed gelatin until dissolved.
Use the dropper (that came with the skulls) to fill at least 12 skulls and place them in the fridge. You’ll have enough to fill more than 12 - it’s a good idea to fill more because some may come out imperfect. If you still have extra, simply use it in the next step.
Place cube molds on a small cutting board for easy placement in the fridge. Bloom another 1½ tsp. Knox Gelatin in 1 tbsp. cool water. Heat ¼ cup white chocolate liqueur in a small saucepan for no more than 1 minute over low/medium heat. As soon as you see any boiling bubbles, turn the heat off. Stir in bloomed gelatin until dissolved.
Spoon 1 tsp. liqueur/gelatin into each cube mold. Refrigerate for 1 hour.
After 1 hour, repeat step 4 with ¼ cup Mr. Black liqueur (instead of white chocolate). Spoon 1 tsp. liqueur/gelatin into each cube mold. Refrigerate for 1 hour.
Repeat steps for another white chocolate liqueur layer and Mr. Black layer, ensuring each layer chills for 1 full hour before adding another.
Carefully remove the cubes of jello. They should slide out of their molds easily. I find I can flip the cube molds entirely inside out without breaking the jello.
Remove skulls from their molds. Place a skull on each cube and serve.