Place 12 2-ounce plastic ramekins on a baking sheet or in a baking dish. (This makes them easier to transport. Set aside.
Chocolate Layer
In a medium bowl, whisk together gelatin powder and 1/4 cup half-n-half. Let sit for a few minutes until the gelatin is dissolved.
In a small saucepan, heat 1/2 cup half-n-half until just starting to simmer. Pour the hot half-n-half into the bowl with the gelatin whisking to incorporate. Keep stirring until gelatin dissolves. Pour in chocolate syrup and vodka, and stir to combine.
Fill each ramekin 1/2 full with chocolate jello. Chill for 30 minutes in the fridge or until jello has just set up. (It might be slightly tacky to the touch still.)
Mint Layer
In a medium bowl, whisk together gelatin powder and 1/4 cup half-n-half. Let sit for a few minutes until the gelatin is dissolved.
In a small saucepan, heat 1/2 cup half-n-half until just starting to simmer. Pour the hot half-n-half into the bowl with the gelatin whisking to incorporate. Keep stirring until gelatin dissolves. Pour in sugar and crème de menthe, and stir to combine.
Fill each ramekin the rest of the way with mint jello. (Leave a little room at the top so your lid doesn't stick to the shots.) Chill for 2 hours or until set. Serve immediately or top with lids and chill until ready to serve.
Notes
If you want riched jello shots, you can substitute heavy cream for half-n-half.
In place of the vodka, you can substitute white crème de cacao or chocolate vodka if you prefer.
If you want more than 2 layers (as pictured) fill each cup 1/4 full with chocolate jello, chill 20 minutes. Fill to the 1/2 way point with mint jello, chill 20 minutes. Fill 3/4 full with chocolate jello, chill 20 minutes. Top with mint jello and chill 2 hours. It takes a little more time, but the presentation is pretty.