Inverse design techniques in the context of nanophotonics have helped in discovery of compact and counter-intuitive nanophotonic structures. We introduce the concept of spectral domain inverse design to search through the dispersive trade-space (permittivity) of nanocomposite metamaterials. We develop a hybrid optimization technique that combines genetic algorithms and gradient descent methods. We utilize this technique to inverse design an ultra-thin thermophotovoltaic emitter coating material.