Make the yellow cake according to package directions and bake it in the buttered Pyrex dish.
While the cake is baking make the custard
Pour the milk in a saucepan, (at least 4 quarts) add 1/2 cup of sugar, stir and heat on medium until simmering
Separate the eggs (reserve the whites for the Italian meringue) place the egg yolks in a bowl add 1/2 cup of sugar and cornstarch WHISK TOGETHER IMMEDIATELY or the egg yolks will crystallize
TEMPER THE HOT MILK INTO THE EGG YOLKS
Turn the heat off on the milk. Carefully and slowly whisk a cup of hot milk into the egg yolks until well blended. Repeat with another cup of milk.
Pour the egg yolk mixture into the remaining hot milk and stir over medium heat until thickened and it begins to bubble. Be careful not to scorch the custard.
This should take about 5 minutes
Slice the banana and scatter the banana slices over the cake
Pour the custard over the bananas and cake
MAKE THE ITALIAN MERINGUE
Combine 1 cup of sugar, corn syrup and water in a small sauce pan.
Cook until sugar syrup registers 240 degrees on an instant read thermometer
With the whisk attachment beat the egg whites, cream of tartar and salt in the stand mixer on medium speed until thick and fluffy, about 2 minutes
With the mixer running, carefully and slowly pour the hot sugar syrup down the side of the mixer
Continue to whisk for another minute or two until the meringue is thick and fluffy
Top the custard with the meringue. Toast the meringue carefully in the oven at 400 degrees for 5 minutes or use a culinary blow torch (do this at your own risk)