This one has three distinct layers. I can guarantee that this will layer nicely if you use half-and-half. If you want a lighter drink, you could use milk, but the layers might not hold as well - but then again, you're going to stir it, anyway.
The flavor reminds me a little bit of butterscotch - but a nutty butterscotch.
Creamy Hazelnut and Rum
1 ounce hazelnut liqueur
1 1/2 ounces half and half
1/2 ounce rum
Fill a small glass with ice. Add the hazelnut liqueur, then the half-and-half. Float the rum on top. Serve.