Developer for Mac OS and Windows cross platform app

I am in need of an experienced developer who can design a cross-platform application for Mac OS and Windows, who is proficient in user interface design and API integration. The software's user interface needs to have a contemporary, modern look that appeals to users. Furthermore, the software should be seamlessly integrated with an existing web API for efficient functionality. Hence, your expertise should include:
- Proficient in Mac OS and Windows software development
- Expertise in modern user interface design
- Experience with API integration, particularly with existing web APIs.