Preheat your oven to 350 degrees. In a large pot combine 1/3 cup sugar, cornstarch, and salt. Stir until blended. Add milk. Turn your element up to medium heat. Stir continuously until thickens and boils. Only boil for about a minute. No longer. Remove from heat.
Grab a small bowl for your egg yolks. Whisk in about 1/2 cup of the hot custard mixture. After blended pour back into the rest of the custard in your pot. Cook over medium heat for a couple minutes, again stirring constantly, for about 2 minutes. Add butter and vanilla. Stir until incorporated.
Prepare an 8x8 or 9x9 inch pan with cooking spray. Place wafers as evenly as possible covering the bottom of the pan. You will have to cut some in half. That is ok. Just make sure the bottom is covered as best you can.
Next place a layer of sliced bananas on top. Then, using a ladle, scoop custard over top of bananas until covered. Then place more wafers on top of the custard. Don't worry if your wafer pattern is different than the first one. Just make sure to place enough wafers to create a layer. Place remaining sliced bananas on top of the wafers. Dump remaining custard all over the bananas, right to the edge to prevent shrinking. Set aside.
In a large bowl beat egg whites and 1/4 cup sugar until frothy on low speed. Add cream of tartar and turn your speed up to medium. Gradually incorporate the remaining 1/4 cup of sugar. Beat until stiff peaks form. Once this happens, immediately drop spoonfuls of the meringue over top of the custard. Make sure the meringue touches all of the edges of the pan to help prevent it from shrinking. Bake for 20 minutes. Meringue will be golden brown. Remove from your oven and allow to cool on a wire rack for an hour. Then place in your fridge for a minimum of 4 hours before it is ready to serve! Enjoy!