Whatever framework we are going to build it in.
So, for instance, with the Balsamiq Wireframes Editor, we wireframed the basic look of the editor in Mockups. Then we had Mike take that structure and finalize the actual look and feel of the design in Sketch. We took those designs and started building the actual app.
The first version of the Wireframes editor was literally just the canvas and the spaces where things would live. Over time, the team added components, menus, controls, and eventually built out the whole app. That’s how we built Cloud, and that is how we are building the desktop apps. We aren’t convinced that going to a prototype and then to code makes sense as you are kind of doing things twice.
However, if that is what makes sense to other folks, we totally understand. For me, design is figuring out the process that makes the most sense to you, and then finding the tools that are best suited to that process.