A popular Starbucks coffee variety allegedly contains undeclared potassium — potentially harming clueless customers who believe they’re chugging pure java, The Post has learned. A complaint filed late last month with the North Carolina Department of Justice’s Consumer Protection Division claims Starbucks Dark French Roast Coffee “significantly exceeds” levels of the essential nutrient found in other unadulterated beans sold on supermarket and retail shelves. Independent laboratory testing found 13% more potassium in Starbucks’ dark roast compared to its house blend as well as Dunkin’ Donuts’ and Lavazza’s dark roast varieties, according to the complaint.