Preheat oven to 375 degrees F. Generously grease cups of popover pan or eight 6- to 8-ounce ramekins. In blender, puree eggs, milk, flour, unsweetened cocoa, sugar, butter and salt until smooth. Divide evenly among cups. Bake 40 minutes. With small pairing knife, cut small slit in top of each popover. Bake 10 minutes more. Remove from oven; immediately transfer from cups to wire rack. Serve warm. Cooled popovers can be kept at room temperature up to 3 hours or frozen up to 1 month. Reheat at 350 degrees F until crisp.