Make the Vanilla Custard:

1. Combine dry ingredients: In a medium heavy-bottomed saucepan, whisk together the sugar, flour, and salt until well combined with no lumps.
2. Add milk gradually: Slowly pour in the milk while whisking constantly to prevent lumps from forming. The mixture should be smooth and well incorporated.
3. Cook the custard: Place the saucepan over medium heat and cook, stirring constantly with a whisk or wooden spoon, until the mixture thickens and comes to a gentle boil. This will take 8-10 minutes. The custard should coat the back of a spoon when ready.
4. Temper the egg yolks: Remove the saucepan from heat. In a small bowl, quickly whisk the egg yolks. Slowly add about 1 cup of the hot custard mixture to the yolks while whisking constantly (this is called tempering—it prevents the eggs from scrambling).
5. Finish the custard: Pour the tempered egg mixture back into the saucepan with the remaining custard, whisking constantly. Return to medium-low heat and cook for 2-3 more minutes, stirring constantly, until the custard is thick and creamy.
6. Add butter and vanilla: Remove from heat and stir in the butter and vanilla extract until the butter is completely melted and incorporated. Place a piece of plastic wrap directly on the surface of the custard to prevent a skin from forming. Let it cool for about 15-20 minutes.

Assemble the Banana Pudding:

7. Prepare the bananas: Slice the bananas into 1/4-inch rounds. Gently toss them with lemon juice to help prevent browning.
8. Create the first layer: Spread a thin layer of the warm (but not hot) custard on the bottom of a 9×13-inch baking dish. Arrange a single layer of vanilla wafers over the custard, covering as much of the bottom as possible.
9. Add bananas and custard: Layer half of the banana slices over the vanilla wafers. Pour half of the remaining custard over the bananas, spreading it evenly to cover.
10. Repeat layers: Add another layer of vanilla wafers, then the remaining banana slices, and finally the remaining custard. Spread the top layer of custard evenly and smooth it out.
11. Chill: Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight. This allows the flavors to meld and the vanilla wafers to soften beautifully.

Make the Whipped Cream Topping:

12. Whip the cream: Just before serving, pour the cold heavy cream into a large mixing bowl. Using an electric mixer on medium-high speed, beat the cream until it begins to thicken. Add the powdered sugar and vanilla extract, then continue beating until stiff peaks form. Be careful not to overbeat or you’ll end up with butter!
13. Top and serve: Spread or pipe the whipped cream over the chilled banana pudding. For an extra touch, garnish with crushed vanilla wafers and banana slices. Serve immediately and watch it disappear!