The scene I am particularly thinking about is Darcy coming over to save Bruce from some people who were starting to upset him, and them dismissing her as unimportant/just a ditzy girl. She ends up talking science/math to Bruce, weirding out the other people and calming Bruce down. They escape from those people and Darcy make a joke about what she said, and Bruce talks to her about how she really is smart, and that she is actually a co-author or contributor to a lot of papers published by Jane, Bruce, and Tony because of all the work that she does for them. Cue her being shocked, but yea Darcy is an intelligent bamf!
98% certain this would be on AO3; no idea if there is a pairing or what it might be, and again, no idea if it was a one-shot or a scene in a long fic. Any help is greatly appreciated. Thanks!