Producing organic shapes in concrete has been a challenging problem since complex freeform buildings became a major trend in contemporary architecture. Many different techniques for casting doubly-curved shapes have been proposed. Most of them produce elements which exactly match a preconceived design, but at a high cost in manufacture. Fabric formwork techniques (such as those pioneered at the Centre of Architectural Structures and Technology at the University of Manitoba (CAST)) are relatively economical, but require a form-finding approach which takes into account the physics of casting, as well as structural and functional requirements of the finished element. The research presented here involves a specialised methodology for the design and manufacture of optimised concrete elements cast in fabric formwork. Using a novel software tool, our approach lies in between the largely intuitive methods reported by CAST and the precise but expensive computer-controlled manufacturing methods normally used in practice. Combining topological optimisation with computational form-finding, the developed software guides the designer towards a shape that is economical in both material and manufacturability. By combining knowledge of computational structural analysis, optimisation algorithms, fabric simulation and the practical casting techniques of fabric formwork; the gap between structurally optimised forms, and those developed intuitively by fabric casting, can be bridged. This is demonstrated through a case study involving the computational design of a centrally supported slab, supplemented with design studies realised using plaster scale models.