A number of studies have now confirmed that dark chocolate and cocoa flavanols improve endothelial function.
Endothelial dysfunction is a systemic pathological state of the endothelium (the inner lining of blood vessels) and can broadly be defined as an imbalance between vasodilating and vasoconstricting substances produced by (or acting on) the endothelium.
Reduced nitric oxide bioavailability with endothelial dysfunction is considered to be one of the first inital steps in the pathogenesis of atherosclerosis and is also considered to be an independent predictor in the future development of cardiovascular risk and events.
Dark chocolate and cocoa is an excellent dietary source of flavonoids, specifically flavanols, also called flavan-3-ols.
Research has found that dark chocolate and cocoa flavanols may possess a number of cardiovascular related health benefits including reducing blood pressure, improving metabolic health/insulin resistance and endothelial function.
Sugar content may counteract the potential metabolic and cardiovascular health benefits of dark chocolate and cocoa flavanols.
Choose sugar-free dark chocolate with a high cocoa content(70%+) and/or a high flavanol cocoa powder such as Aduna’s High Flavanol Super Cacao Powder.
1. Dark Chocolate Improves Flow-Mediated Dilatation (FMD)
A systematic review, meta-analysis, and dose–response analysis of randomized controlled trials published in 2019 investigated the effect of dark chocolate on flow-mediated dilatation (FMD) using randomized clinical trials.
The flow-mediated dilation test is now the most commonly used noninvasive assessment of vascular endothelial function in humans.
The systematic review of randomized clinical trials concluded:
Current evidence indicated the beneficial effect of acute and chronic consumption of dark chocolate and flavonoids on FMD. Non-linear associations should be considered when investigating the effects of dietary intakes on FMD. 
2. Dose-Response Relationship Between Cocoa Flavanols & Endothelial Function
A systematic review and meta-analysis of randomized trials publiched in 2019 examined the dose-response relationship between cocoa flavanols and human endothelial function.
The systematic review and meta-analysis concluded:
This meta-analysis provides evidence that cocoa flavanols could significantly improve endothelial function, with an optimal effect observed with 710 mg total flavanols, 95 mg (-)-epicatechin or 25 mg (+)-catechin. 
The information in this article has not been evaluated by the FDA and should not be used to diagnose, cure or treat any disease, implied or otherwise.