Paper prototyping is commonly used to identify usability problems in the early stages of user interface design, but it is not very well suited for the evaluation of mobile interfaces. The reason is that mobile applications are used in a rich realworld context, which is hard to emulate with a paper prototype. A more powerful technique is to test the design on a mobile device, but building a functional design prototype requires much more effort. In this paper, we try to get the best of both worlds by building interactive prototypes with a digital pen. We developed a system which allows for sketching a user interface on paper and manually associating the interface elements with functionality. This enables designers to bring their design ideas to paper without any restrictions, define the meaning of selected interface elements, and test them on a mobile device instantaneously. We conducted a user study in which the participants had to design and test a small application with our system. The results provide evidence for the feasibility and positive aspects of our approach, but also showed some limitations and missing functionalities of its current implementation.