Back when I was 18 and living in a group house with a couple of (male) friends, the absolute height of decadence was a Saturday night at home, without the housemates but with my best friend Susan, swooning over Booker, eating chocolate (preferably dark and peppermint), and drinking very small glasses of Kahlua.
To this day the taste of straight Kahlua - the delightful sensation of it just slightly burning my mouth, my throat - spells decadence to me.
I don't drink it much because, well, I don't drink much alcohol in general, and besides it's expensive and I am a penny pincher. So one bottle will last me a year or more. But when I do partake a little, it takes me straight back to that feeling of slightly forbidden luxury. Delicious.