Your corner "spalled" which is the definition of what happens when excess water works it's way into concrete and the freezing action blows the surface off. Clean both surfaces with a wire brush to get any loose material off. Coat the back of the loose chunk with a generous layer of "PL Premium construction Adhesive" and push I back in place. This is available in caulk tubes at the hardware store/ big box lumber yard. Drive a small stake of furring strip or whatever, into the yard about two feet out from the corner. Use another piece of wood as a prop to keep the corner tight, drive it in between the stake and the corner, to keep pressure on the wedge. Once the glue sets, caulk the top edge of the damaged area to eliminate water penetration.
With the right tools and material, this is a half hour fix that will last forever. DON'T get involved with trying to use tapcons, making a patch out of mortar, or finding some concrete mix that will make your missing chunk stick to the corner. It's a waste of time. Ever notice home many new buildings have stone veneer on them? This is a thin layer of "manufactured stone" that is nothing more than fancy concrete lumps, mortared to a stuccoed wall. What to know what happens when the mason who did the work gets a call that a stone fell off? He grabs a caulk gun, a tube Of PL, an glues it right back where it was. You can do the same thing. Good luck.