Whisk 1/2 cup sugar, flour, salt, milk and egg yolks in a saucepan and cook over medium heat for 8 to 10 minutes, whisking, until thick and 170°F; stir in the vanilla.
Arrange one-third of the wafers and bananas in a 2-quart baking dish and cover with one-third of the warm custard; repeat the layers twice.
Beat the egg whites with the cream of tartar to soft peaks, then add the remaining sugar gradually and beat to glossy stiff peaks.
Spread the meringue over the hot pudding all the way to the dish edges.
Bake at 375°F for 8 to 10 minutes until browned, cool for 1 hour and refrigerate for at least 3 hours.