Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
WhyGrasp of the Undying
Grasp on Malphite allows you to trade effectively with auto attacks in lane. Each point of health is especially important/strong on Malphite thanks to having so much natural Armor.
Shield Bash synergizes well with Malphite's passive and looking to trade with empowered autos from [W].
If you manage to make it through the early game, Conditioning's bonus resists make a monster and incredibly difficult to kill in the mid game.
Overgrowth's bonus health scales incredibly well with Malphite's natural Armor stacking. Each point of health you earn from Overgrowth is that much stronger thanks to your resists.
Manaflow Band allows you to consistently harass in the lane phase with [Q] thanks to the point-and-click proc.
Malphite's natural build consists of a lot of Cooldown Reduction items and Transcendence allows you to overcome any fear of over-capping CDR. The extra bit of AP you'll get from over-capping is welcomed.