This is second-hand, never tried this myself
as I stay out of the sun in order to avoid sunburn.

But an acquaintance who does the odd landscape assignment, and ends up sunburned, has a recommendation for severe sunburn.

It's that high-powered ointment/creme for severe diaper rash,
the kind that cools the baby's skin down so fast that
you can see vapor rise off the treated skin.
He uses the diaper-rash formula for his sunburns.