Check if the valve stem or head is bent, if you hit the piston you normally put a bend in it. Been their done that
A small bend can lead to exhaust gas leakage past the valve and seat. These hot gases can rapidly burn out the valve like you appear to have.
Clean the valve and check the face of the fracture with missing bit. If its a fracture that will be obvious versus a oxidation burnt surface.
Even a small leak due to wear and without hitting the piston ( or other valve) and bending the valve can lead to a valve burning out rapidly. Lean mixtures and over advanced ignition and loose exhaust stem clearances and poor conductivity valve guides can also contribute to or cause the problem especially in a race engine with constant full throttle high rev use.
cheers
Rohan