Preheat oven to 350 degrees. Lightly spray a 9"x 13" pan with vegetable spray and set aside. In a large bowl, add the milk, sugar, egg yolks, vanilla and salt. Using a hand mixer, mix all ingredients together. Set aside. Add the cubed bread to the pan and then the milk mixture. Place the pan in the oven and bake for about 1 hour and 20 minutes. The bread pudding is ready when a knife inserted comes out clean. Be sure to keep an eye on it once it gets close to the 1 hour and 15 minute mark. You should start checking if it's done at this point. You can now prepare the meringue while the bread pudding is baking.
Add to the bowl of a stand mixer, using the whisk attachment, or to a large bowl if using a hand mixer, the egg whites that have been reserved. Along with the egg whites, add to the bowl the cream of tartar and the vanilla. It is best to use clear vanilla if possible so as to keep the meringue white in colour. Start the hand or stand mixer on low speed. Once the egg whites start to get frothy and opaque, begin adding in the sugar in small increments. Leave the mixer running as you do this. The egg whites will start to turn very white with a pillowy texture. Keep on mixing!! You are trying to achieve a consistency that has stiff peaks that will hold their shape. The egg whites will take on a glossy appearance. It will be smooth and silky with no sugar grains evident. Once you have reached this point, the meringue is ready to go on top, It may take quite a bit of time to reach this point, especially if you are using a hand mixer. This can take upwards of 10 minutes. To make the bread pudding look attractive, create small peaks with your spreader before returning the pudding to the oven. You'll want to bake for another 15-20 minutes until the meringue is a nice golden colour. I have included a picture below demonstrating what the colour should look like. Remove the bread pudding from the oven and let cool before slicing and eating!