Buffon, making his 620th appearance in the league, denied Diego Farias after referee Fabio Maresca used VAR to award the spot-kick for an Alex Sandro foul.
Juventus were already leading at that point through Mario Mandzukic's volley.
Paulo Dybala swept in a second for the hosts before Gonzalo Higuain struck.
Juventus are looking to record a seventh straight Serie A title this season.
Match ends, Juventus 3, Cagliari 0.
Second Half ends, Juventus 3, Cagliari 0.
Attempt missed. Paulo Dybala (Juventus) left footed shot from the centre of the box is close, but misses to the left. Assisted by Stephan Lichtsteiner.
Foul by Giorgio Chiellini (Juventus).
João Pedro (Cagliari) wins a free kick in the defensive half.
Attempt blocked. Mario Mandzukic (Juventus) right footed shot from outside the box is blocked. Assisted by Blaise Matuidi.
Foul by Mario Mandzukic (Juventus).
Marco Andreolli (Cagliari) wins a free kick in the defensive half.
Attempt saved. Paulo Dybala (Juventus) left footed shot from outside the box is saved in the centre of the goal. Assisted by Blaise Matuidi.
Corner,  Juventus. Conceded by Marco Andreolli.
Attempt blocked. Paulo Dybala (Juventus) left footed shot from outside the box is blocked. Assisted by Douglas Costa.
Hand ball by Nicolò Barella (Cagliari).
Attempt blocked. Mario Mandzukic (Juventus) right footed shot from outside the box is blocked. Assisted by Douglas Costa.
Substitution, Cagliari. Daniele Dessena replaces Luca Cigarini.
Substitution, Cagliari. Marco Sau replaces Duje Cop.
Corner,  Juventus. Conceded by Paolo Faragò.
Foul by Miralem Pjanic (Juventus).
João Pedro (Cagliari) wins a free kick in the defensive half.
Stephan Lichtsteiner (Juventus) wins a free kick on the right wing.
Foul by Artur Ionita (Cagliari).
Foul by Stephan Lichtsteiner (Juventus).
Artur Ionita (Cagliari) wins a free kick on the left wing.
Stephan Lichtsteiner (Juventus) is shown the yellow card.
Foul by Stephan Lichtsteiner (Juventus).
Artur Ionita (Cagliari) wins a free kick in the defensive half.
Substitution, Juventus. Douglas Costa replaces Juan Cuadrado.
Substitution, Juventus. Blaise Matuidi replaces Gonzalo Higuaín.
Corner,  Juventus. Conceded by Alessio Cragno.
Attempt saved. Gonzalo Higuaín (Juventus) right footed shot from a difficult angle on the right is saved in the centre of the goal. Assisted by Juan Cuadrado.
Goal!  Juventus 3, Cagliari 0. Gonzalo Higuaín (Juventus) left footed shot from the left side of the box to the bottom right corner. Assisted by Alex Sandro.
Offside, Juventus. Juan Cuadrado tries a through ball, but Stephan Lichtsteiner is caught offside.
Attempt blocked. Artur Ionita (Cagliari) right footed shot from the centre of the box is blocked.
Substitution, Cagliari. João Pedro replaces Simone Padoin.
Foul by Alex Sandro (Juventus).
Diego Farias (Cagliari) wins a free kick in the attacking half.
Substitution, Juventus. Sami Khedira replaces Claudio Marchisio.
Attempt missed. Paolo Faragò (Cagliari) left footed shot from the centre of the box misses to the left.
Foul by Alex Sandro (Juventus).
Diego Farias (Cagliari) wins a free kick on the right wing.
Foul by Alex Sandro (Juventus).
Gianluigi Buffon saved the first penalty awarded by video assistant referee (VAR) in Serie A as Juventus began their title defence with a comfortable win against Cagliari.