In this paper, we present a novel approach to create devices with tangible user interfaces by physical com-position. While the separation of the user interface from the application logic has a long tradition in software engineering, for products with tangible user interfaces there is no equivalent approach that realizes a true separation and flexible combination of interface components, underlying technology, and software parts. We propose a novel concept that is based on an inner Core for the basic technical and software platform of a product and an outer Shell that builds a flexible and ex-changeable tangible user interface from passive components. Using vision-based tracking, we can realize a clear separation between the components. No wiring is necessary. This paper introduces our novel approach and presents a first working prototype as well as initial results from its application in a design workshop.