In a medium-size mixing bowl, whisk together the flour, sugar and salt. 

Add the egg yolks (save the whites for a meringue) and ½ cup of the milk, whisking again until smooth and combined.

Pour the remaining milk into a heavy saucepan and warm over medium heat, just until bubbles begin to form around the edges of the pan.  

When the milk is hot, slowly pour 1 cup of it into the egg yolk mixture, whisking until combined to temper the egg yolks.  Then transfer the tempered yolks to the pan with the remaining hot milk, whisking constantly to combine.  

Continue to cook and stirring constantly over medium heat for about 10-12 minutes until the pudding has thickened.  You may prefer to use a double boiler to prevent scorching, but I've never had a problem as long as I keep my heat at medium and stir constantly.  

Once the pudding is thick, remove from the heat and stir in the butter and vanilla.  

Layer Nilla wafers in the bottom of individual serving dishes or a large bowl, trifle dish, pie plate, or baking dish.  Add half of the sliced bananas.  Spoon half of the pudding over the bananas, then repeat the layers with the remaining Nilla wafers, bananas, and pudding. 

Beat the reserved egg whites and cream of tarter in the bowl of a stand mixer fitted with a whisk attachment until soft peaks begin to form.  Add the granulated sugar while beating and continue to beat until stiff peaks form, about 3-5 minutes.  Spread the meringue on top of the banana pudding, using the back of a spatula to create peaks and swirls in the meringue.

There are a couple of ways to finish the meringue depending on what container or presentation you are using for your banana pudding.  If I'm making a big bowl or casserole dish of pudding, I will bake it for 10-12 minutes at 350 degrees F until lightly browned on top.  For individual servings in mason jars or ramekins, I will broil them until the top of the meringue is toasted.  Garnish with Nilla wafers, if desired.