This is a cross-sectional survey of over 13,000 US adults. 
The study compared self-reported chocolate consumption with depressive symptoms. Eating chocolate was associated with less risk of clinical depression. Non‐dark chocolate consumption was not significantly associated with clinically relevant depressive symptoms. People who ate dark chocolate in the past 24 hours were 70% less likely to report depression. The consumers of dark chocolate ate only 12 grams a day, a little less than half an ounce. The cut-off for “dark” chocolate was ≥ 45% cocoa.
1 Jackson SE, Smith L, Firth J, et al. Is there a relationship between chocolate consumption and symptoms of depression? A cross-sectional survey of 13,626 US adults. Depress Anxiety 2019;36:987–95. doi:10.1002/da.22950