Step 1: In a large saucepan, whisk together 4 cups milk, 1 cup granulated sugar, all-purpose flour, and a pinch of salt until smooth. Bring to a boil over medium heat, whisking constantly. Reduce heat and simmer for 1-2 minutes until thickened.
Step 2: In a separate bowl, lightly beat the 6 egg yolks. Slowly temper the egg yolks by gradually whisking about 1 cup of the hot milk mixture into the yolks. This prevents the eggs from scrambling.
Step 3: Pour the tempered egg yolk mixture back into the saucepan with the remaining milk mixture. Cook over medium-low heat, stirring constantly, for 2-3 minutes until the custard thickens further. Do not boil.
Step 4: Remove from heat and stir in the cubed unsalted butter and vanilla extract until fully melted and combined. Set aside to cool slightly.
Step 5: Preheat oven to 350°F (175°C). In a 9x13 inch baking dish, spread a thin layer of custard on the bottom. Arrange a single layer of vanilla wafers over the custard, followed by a layer of sliced bananas.
Step 6: Repeat the layering: custard, vanilla wafers, and bananas, ending with a generous layer of custard on top. Reserve some wafers for garnish if desired.
Step 7: For the meringue, in a clean, dry bowl, beat the 6 egg whites with an electric mixer until foamy. Add the remaining ½ cup granulated sugar gradually, along with the cream of tartar (if using), and continue beating until stiff, glossy peaks form.
Step 8: Spread the meringue evenly over the top of the casserole, ensuring it touches the edges of the dish to prevent shrinking.
Step 9: Bake in the preheated oven for 20-25 minutes, or until the meringue is golden brown. Watch carefully to prevent burning.
Step 10: Remove from oven and let cool at room temperature for at least 30 minutes, then refrigerate for at least 4 hours, or preferably overnight, before serving. This allows the custard to set and the wafers to soften beautifully.