Preheat oven to 375 and adjust a rack to be in the middle of the oven.
Find a a medium sized pot that the bowl to your stand mixer can easily rest on top of, to use as a double boiler. (maybe that pot you used to make the custard? Wash it out while you're washing that stand mixer bowl) Fill that pot with about 3 inches of water, and test that your bowl can fit on top without touching the water. Put the pot on medium-high heat and let come to a simmer.
Whisk together egg whites, sugar, salt, cream of tartar and vanilla, then put on top of simmering water. Whisk continually over heat until mixture gets to 175 degrees. Sugar should be dissolved, and mixture should look less like snot. Take bowl off double boiler, and put into your stand mixer with the whisk attachment.
Whisk on high speed until glossy, voluminous and barely warm to the touch. It will look like shiny clouds.
Immediately pour meringue on top of custard. Spread around with spatula, and, if desired, use a fork to twirl and prick the meringue. You could also choose to pipe the meringue on top of the pie, or mound it all on top like a pyramid. Have a fun time!
Place pie on a sheet pan, and bake in preheated oven for about 20 minutes. Meringue should be toasty and golden brown all over (a few darker spots can be exciting.) Remove from oven, and allow to cool to room temperature before placing in fridge. Pie should be served entirely chilled.