Hardtack is more typically eaten, of course, when cold and hard as an oak shingle. A good, well-tempered piece of hardtack should not shatter, but suffer chipped edges only, when hurled against a wall or when dropped onto a stone floor. The holes (reputedly and jokingly) served as lacing points for stitching pieces together to form a bullet proof vest, like Roman armor.
Hardtack was the most important and common of the Holy Trinity of the Civil War soldier's diet: salt pork, hardtack and coffee – and the three were often combined. Period preparations for softening hardtack to render it chewable included soaking it your coffee until it acquired the taste and consistency of wallpaper paste; or crumbling it into your salt pork grease, after smashing it with a musket butt or pistol handle, to form a greasy mush. Only those daring and reckless few, with large mouths and perfect teeth and the willingness to risk them in an era of rampant tooth decay, took on the corner of a piece of hardtack without some initial softening preparation.
The similarity between Civil War hardtack (which requires endless chewing to soften) and baby's teething biscuits (which are of the same size, shape, taste and consistency) is so obvious, in fact, that the potential for a direct relationship needs investigation. I might take a slight difference with the inclusion of vegetable fat in the recipe below, as the correct period term for this component of the recipe is LARD.