Prepare a double boiler. Mix together 1/2 cup sugar, egg yolks, flour and salt. Add milk and mix again. Heat on stove until the custard sticks to the sides of pan, approximately 20 minutes. Add vanilla after mixture has thickened. Let cool.
To assemble: Put few drops of pudding in the bottom of a 2-quart dish and spread around. Layer Nilla Wafers and cut up 2 bananas, add 1/2 of custard, repeat layer and arrange wafers around the sides.
In a separate bowl beat egg whites with 1/4 cup sugar and cream of tartar until it forms stiff peaks, spread on pudding and bake in a preheated 425 degree F oven for 4 minutes, just brown on top.
Serve warm or cold.