Can take Ignite against low burst teams.

Unlike most supports, Miss Fortune support rushes Eye of the Watchers for the incredible gold it provides her due to Make it Rain being able to proc all stacks every time they're up, safely, from long range. This allows support Miss Fortune to quickly scale, build some damage and utility, and provide massive team fight AoE, as a support, while still providing utility through Executioner's Calling and Black Cleaver debuffs.

Miss Fortune support can be built with either a focus of physical or magic damage, depending on what your team needs more of. Physical is generally stronger, however. Only go magic if you're the only real magic threat.

Items:
Eye of the Watchers: Gives Miss Fortune nearly the income of a carry, and damage on Make it Rain in lane, as well as mandatory ward coverage.
Executioner's Calling: debuffs the healing of the entire enemy team in team fights, and builds into Mortal Reminder late game.
Black Cleaver: debuffs the armor of the entire enemy team in team fights, and provides excellent CDR.
Standard Items:
Boots of Swiftness allow Miss Fortune to kite the enemy around and re-position quickly to get the perfect angles for Bullet Time.
Youmuu's Ghostblade is a luxury damage item, but Miss Fortune support can usually afford these. Provides exceptional damage in synergy with Black Cleaver. If you're not planning on building this or another CDR item, feel free to pick up Ionian Boots of Lucidity instead of Boots of Swiftness to cap CDR.
Duskblade of Draktharr is another luxury damage item, but allows Miss Fortune to kill off single targets as well as deal AoE.
Edge of Night: a Duskblade of Draktharr alternative, if you're getting disrupted during Bullet Time, or are scared of magic damage.
Niche Items:
Locket of the Iron Solari, if your team needs protection from AoE more than it needs additional AoE damage, pick this up.
Sterak's Gage if you're the target of assassins for some reason, consider this.
Maw of Malmortius if you're the target of mage assassins for some reason, consider this instead of Youmuu's Ghostblade.
Guardian Angel: If you really want defense.
Zz'Rot Portal: If you need help holding gates.
Magic Items:
Liandry's Torment is the real additional source of magic damage you'll get on a magic focused support Miss Fortune. Nothing else substantially buffs her magic damage output.
Ionian Boots of Lucidity fit a little better here than on the AD build, as you're less likely to get 10% CDR elsewhere.
Mobility Boots are a decent choice with this build, in that you'll be spending slightly more time trying to catch people with Make it Rain, as it provides relatively more damage with this build. Make sure to pick up 10% CDR somewhere else if you pick this build.
Void Staff will compound your Make it Rain most substantially after Liandry's Torment.
Rylai's Crystal Scepter is the better option if the enemy isn't building any magic resist, as it will apply a small slow to amplify the Liandry's Torment burn on Bullet Time.
Zhonya's Hourglass is a decent CDR AP option, as it can also protect you from assassin threats.
Abyssal Mask is similar to Zhonya's Hourglass in function, although more suited to countering heavy magic damage teams. The passive is mostly wasted, as you won't go this rout with other magic damage allies.