We develop and apply an optimization method to design unidirectional invisibility cloaks. Our method is based on minimizing the forward scattering amplitude of the cloaked object which, by the optical theorem, is equivalent to the total cross section. The use of the optical theorem circumvents the need to evaluate and integrate the scattering amplitude over an angle at each iteration, and thus provides a simpler, more computationally efficient objective function to optimize structures. We implement the approach using gradient descent optimization and present several gradient-permittivity unidirectional cloaks that reduce scattering by metallic targets of different sizes and shapes.