Second-placed Napoli made sure the gap stayed at three points, however, with the league's top scorer Edinson Cavani netting a penalty early in the second half and bagging a second seven minutes from time to secure a 2-0 win at AS Roma.
The unplayable Uruguayan, who managed his first after error-prone Brazil defender Juan had brought down Marek Hamsik, took his league tally to 20 having been accused of not being an out-and-out goal scorer when he arrived in July from Palermo.
Stunned Napoli fans, whose only title success came with Diego Maradona in 1987 and 1990, are in dreamland given no one expected a scudetto challenge this term while seventh-placed Roma are now all but out of the race.
Milan's victory was the perfect warm-up for Tuesday's Champions League last 16 first leg against Tottenham Hotspur even if Cassano and midfielder Mark Van Bommel are ineligible.
Clarence Seedorf, starting in place of the half-rested Robinho, opened the scoring on eight minutes when he rounded the goalkeeper after being picked out by Zlatan Ibrahimovic.
Italy striker Cassano then netted his first goal since arriving from Sampdoria in early January when he exchanged passes with Gennaro Gattuso. After the break he set up substitute Robinho for two more.
"Cassano has been great and he also worked hard when we didn't have the ball," Milan coach Massimiliano Allegri told reporters.
"Up to 2-0 we played well then we slowed a bit. But in the second half Robinho and the like were the difference."
The Rossoneri are seeking their first scudetto since 2004 and laboured in recent draws with Lazio and Genoa but were back on song and could have scored more against a Parma side who are sliding down the table.
Cassano, Ibrahimovic and young German midfielder Alexander Merkel, preferred to Mathieu Flamini, all came close to a fifth.
Substitute Alexandre Pato is set to replace Cassano against Spurs with Flamini maybe coming in for Van Bommel given Andrea Pirlo and Massimo Ambrosini are out injured for six weeks.
