You need a log circuit, which is basically the inverse of the exponential circuit.
Two inputs, A and B
Log them both
add them up
take the exponent.
Mathematically, log(A*B) = log(A)+log(B)
so
A*B = 10 ^ (log(A)+log(B))
(assuming a base 10 log but any base will do as long as your exponential is to the same base)
This is how log table or slide rules allowed us to do reasonably accurate multiplication or division before calculators.
There have been analog multiplication circuits that use this principal. It is now much easier to do it all digitally.