The ECS is a homeostatic regulator of inflammation throughout the human body. Your body produces cannabis like compounds that activate the ECS. One of the two most well known is AEA (anandamide).
AEA is a molecule that acts like a neurotransmitter. AEA is also very similar to THC, the main ingredient in cannabis that gives a euphoric effect. AEA (anandamide) was nick named the "bliss" molecule as ananda is Sanskrit for the word bliss.
So how does chocolate impact the ECS? The enzymes (FAAH) degrade AEA are contained within chocolate. FAAH is an enzyme that degrades AEA. When FAAH is not at normal levels, it allows for more AEA to circulate throughout your body to activate the ECS. As a side note, the real benefit is in cacao, not really milk chocolate.
AEA plays an important role in the regulation of appetite, pleasure, and reward. AEA makes people less anxious and more able to forget fearful and unpleasant experiences. This is another reason exercise helps PTSD, as it produces AEA when
at the right intensity level.
I suspect cacao is not a cure for PTSD. However, by blocking the breakdown of AEA it helps to allow your body's self produced AEA to be more beneficial to anxiety and mental health issues.
So, we can see that it is not only the increase of cannabinoids that serve to affect the ECS, but also the chemicals that degrade the endocannabinoids that can affect the ECS.