We report on the design of a dual-energy X-ray absorptiometry (DXA) method applied to Hologic Selenia digital mammography systems. The aim of the project is to quantify breast composition without thickness assumptions. The method is intended to be applied to standard mammography views as well as ex vivo breast samples. We established the calibration and accuracy using novel calibration phantoms. In this paper, we present our method and preliminary results using an excised whole breast sample. Results showed an average accuracy on the measurements to be less than 3% in the determination of the percent density of the breast phantoms. The chemical analysis of the excised breast and the DXA results agreed within 2%.