Recent years have seen a growing recognition of the transformative potential of games for education and training. Supporting the efficient creation of expressive user interface content that is both effective and engaging is critical to the serious game development process. To this end, this paper presents FLARE, an open source toolkit for creating expressive user interfaces. The toolkit leverages the authoring capabilities of Adobe Flash Professional, eg, vector graphics, animation, text, and sound, while integrating seamlessly into the popular Unity cross-platform game engine. Our experience using FLARE suggests that it is an effective toolkit for developing expressive user interfaces for serious games.