A high performing DEMO divertor target mock-up design that uses the thermal break interlayer concept is presented. The design evolved from a previous design of which six mock-ups were designed, fabricated and subjected to high heat flux testing. The new design was generated using optimisation techniques; specifically, software was developed to automatically process the design of experiments data to enable visualisation of the design space. Despite the more challenging geometric constraints of this second phase, this design performs significantly better than that of the previous phase; the strain in the interlayer, which was the dominant damage mode in the phase 1 testing, is reduced by 28%. Four mock-ups of the selected design were manufactured, all of which successfully passed a series of high heat flux testing, including 500 cycles at 20MW/m2. Design optimisation methods are not widely utilised in fusion engineering, their potential benefits, which are demonstrated here on a plasma facing component, could be applied to many other challenging designs.