Endothelial dysfunction is associated with a higher incidence of adverse cardiovascular events in patients with coronary artery disease (CAD). CAD patients also show impaired function and number of endothelial progenitor cells (EPCs, adult stem cells) which circulate in adult blood and contribute to endothelial repair. Clinical studies suggest that endothelial function can be improved in CAD patients by consumption of flavanol-rich cocoa. Yet, the mechanism is not known. It is also not known whether flavanol-rich cocoa provides an additive, positive effect in patients who are already receiving the maximal recommended therapies for risk factor modification. Therefore, the researchers propose to perform an investigator-initiated, randomized controlled cross-over study administering flavanol-rich cocoa or a placebo for two months in CAD patients on optimal medical therapy. An improvement of endothelial function as measured by flow-mediated dilation (FMD) will be the primary endpoint of this study. The researchers propose to also measure determinants of FMD such as microvascular response, inflammatory markers, metabolites of nitric oxide, as well as the number and function of EPCs in the blood. Importantly, detailed food questionnaires and plasma flavanols/metabolites will help to further support a causal link between flavanol-intake and improved vascular function.