No, you treat it as free cash, so a basis of zero, because you didn't actually pay anything for the BCH and it's treated as income when received.
On the other hand, when you sell the BCH, your basis will the FMV when received, so you would only be taxed the second time on the gain in price (if any).l
On the other hand, when you sell the BCH, your basis will the FMV when received, so you would only be taxed the second time on the gain in price (if any).l