Heaven Slices are a magically prepared food originally intended for Val Roan Soul Sentry units on long patrols. They are small, bite-sized dark grey wafers, each one able to stave a user's hunger for up to two days. Queen Azira explains that about 5 years of preparation are required for each batch, and that once the seal on them is broken they quickly lose their potency. Since Chrysalis' interference in the military, they have been in extremely short supply. Nevertheless, she gives approximately three dozen wafers to Rainbow Dash before her journey over the Grand Choke.
Most of them were dissolved by the anti-magic field in the Grand Choke before they could be used.