Hi there - I have kept Cuckoo Marans for many years, and more recently French Marans and Welsummers.
It really depends what you call a "dark chocolate" egg, if you are thinking Bournville chocolate you are unlikely to find birds which regularly produce eggs that colour. The Marans Clubs say colour 4 on the chart is a true Marans egg, 5 and 6 very good, 7 exceptional, and 8 & 9 rarely produced/accidental which is where an egg has stayed in the egg duct for an exceptionally long time and had many extra coats of pigment applied (brown eggs are white shelled eggs with brown pigment painted on just before they are laid)
Welsummers are supposed to lay an egg which is slightly darker than unglazed terracotta earthenware. In my experience they are not very reliable layers, and are sometimes have temperamental pecky personalities, but it all depends on the strain.
Though there are other brown egg layers basically these are the two breeds which are most likely to lay the darker eggs - without doubt I personally prefer the Marans, the cuckoos are generally the better layers, but my small new flock of French Copper Blacks are also proving very satisfactory in the egg producing department.
You are unlikely to be able to source POL at this time of year and it will be hatching eggs, or very young stock from the better breeders as most will only be thinking of starting this year's breeding programme.
This is a piccie of some of last years Cuckoo Marans eggs ready for the inccy with a Chocolate Kinder egg in the foreground.
Can't post next piccie on forum, but click the link to see some of the darkest my girls can produce against the official egg colour chart
http://i749.photobucket.com/albums/xx135/ssc_photos/darkbrownegg_zpsea2bed2f.jpg
