Donnarumma could not prevent Lecce stunning the San Siro
AC Milan were looking for a second consecutive victory in Serie A to get back on track after an awful start to the season.
The match against Lecce had started well for them when Calhanoglu put AC ahead, but in the second period, the visitors equalised.
In the 62nd minute, Babacar made it 1-1 despite Donnarumma's best efforts.
The Italian keeper saved Babcacar's spot kick, but Babcar was first to the rebound and he made no mistake at the second time of asking.