I was at a full-service liquor store yesterday and happened to scan their bitters collection when I saw a cardamom bitters. I had just come from a Penzeys Spices shop where I was sniffing cardamom to figure out what we should have in the kitchen and this seemed like a funny coincidence. So I bought it! It was from Dashfire and I have a couple of their other products, so it felt safe.
I dumped a couple squirts of it onto a small chunk of ice and then topped it with roughly equal parts Cointreau and bourbon. It's delightful! There's the bitters twist that you'd expect but also the obvious scent of cardamom -- particularly during retronasal olfaction (the drifting of the the odors back through your nasal system after the sip).
I strongly suggest this as something worth adding to your bitters collection.