In a bowl combine the sugar and cornstarch. In a large saucepan, beat the eggs for 1 minute and add the sugar mixture, beating until well blended. Stir in the evaporated milk and whole milk, and mix well. Bring the mixture to a boil over medium heat, stirring constantly, and cook until it thickens enough to coat the back of a spoon. Remove from the heat and stir in the butter. Cool the pudding completely in the pan and stir in the vanilla. Line the bottom and sides of 2 loaf pans (8x4x2 1/4 inches) with vanilla wafer cookies and arrange the banana slices over the cookies. Pour half of the cooled pudding into each of the pans. Refrigerate 3 hours before serving.
This can also be made in 8 to 10 (depending on the size) individual parfait glasses.
Or, if you want to take it to a barbecue down the street, make it in throw away aluminum loaf pans and you don’t have to worry about getting your pans back.