I understand throwing away if they don't like it, there is little else to do in that case, short of offering you to have it instead.
When I arrived in Australia, I got horrified with the amount of wastage, especially working in a restaurant, but even my S.O. would just throw away what he couldn't finish in a meal. I came from a family where we weren't able to take food as much for granted, so we would always store whatever we left for the following day. My parents offering to each other and us if they couldn't finish their meal.
However, this same mentality also made me prone to overeat, because I'd rather eat from my S.O. scarps than seeing them going to the bin. Rather than an angry talk, we both opened up about how our actions were rubbing each other. Thanks to that I realized that by eating what wasn't even my own ration I was treating my body like a disposal bin. I had to learn to accept that those meals are up to my love, not me and if he's full he can store or throw them away. Fortunately, they also saw that many times we can just store them away rather than throwing everything away.
Anyway, I am sharing this because my advice is that you talk it over with her, but not from an accusing tone, rather to try to understand better where she is coming from regarding that. Since she was defensive last time about you treating it like a child, maybe a different approach can help. If it is really affecting your wallet, you can try buying a set amount of whatever drinks she waste and another for you, or write down all the wastage you are both generating and calmly explain how that is putting a big strain on the spending. Try to both come up with ideas to minimize the wastage, for example, it might be a good idea to invest in a smaller cups so she doesn't waste as much, or she can agree to just make half cups before bed, etc.
If she is trying something she knows you like, she can agree to store it for you rather than throwing it. If you don't invalidate her choices completely and instead of trying to find a way that works for both of you, hopefully, you can get into a nice arrangement that will make you both feel good about it.