  The study of hot corinos in Solar-like protostars has been so far mostly
limited to the Class 0 phase, hampering our understanding of their origin and
evolution. In addition, recent evidence suggests that planet formation starts
already during Class I phase, which, therefore, represents a crucial step in
the future planetary system chemical composition. Hence, the study of hot
corinos in Class I protostars has become of paramount importance. Here we
report the discovery of a hot corino towards the prototypical Class I protostar
L1551 IRS5, obtained within the ALMA Large Program FAUST. We detected several
lines from methanol and its isopotologues ($^{13}$CH$_{\rm 3}$OH and CH$_{\rm
2}$DOH), methyl formate and ethanol. Lines are bright toward the north
component of the IRS5 binary system, and a possible second hot corino may be
associated with the south component. The methanol lines non-LTE analysis
constrains the gas temperature ($\sim$100 K), density
($\geq$1.5$\times$10$^{8}$ cm$^{-3}$), and emitting size ($\sim$10 au in
radius). All CH$_{\rm 3}$OH and $^{13}$CH$_{\rm 3}$OH lines are optically
thick, preventing a reliable measure of the deuteration. The methyl formate and
ethanol relative abundances are compatible with those measured in Class 0 hot
corinos. Thus, based on the present work, little chemical evolution from Class
0 to I hot corinos occurs.
