Before beginning, divide the banana slices in half BUT reserving a handful of slices for the end to garnish the dessert. Also set aside a handful of Nilla wafers, whole or crushed, to garnish the dessert.
Fill a wide, shallow bowl or baking dish with equal parts ice and water. I find placing the bowl on top of a folded paper towel or dish cloth will help to absorb condensation as well as keep the bowl from sliding around. Set aside.
Whisk the milk, eggs, sugar, and flour together in a semi-deep sided saucepan and cook over medium-low heat, whisking constantly, for about 20 minutes. The mixture will start out watery and will end moderately thickened. Cut in the butter and the banana extract, stirring until the butter has melted. Remove from the heat.
Place the saucepan in the prepared ice bath and let it stand, whisking occasionally for 10 to 15 minutes, or until the mixture has thickened and thoroughly chilled. NOTE: scrape the pudding into a bowl and apply plastic film directly to the surface of the pudding. This will prevent the mixture from developing a skin. Chill for at least 3 hours. Continue with assembly no more than 1 hour prior to serving.
Cut the bananas into 1/4-inch-thick slices and toss to coat well with the lemon juice.
Line the bottom of a pedestaled trifle dish or glass serving bowl with wafers. You may either crush them lightly or leave whole. Cover with half the custard. Use a rubber spatula to 'guide' the custard to the sides of the bowl. Make a decorative band using some of the Nilla Wafers and go around the bowl, tucking each cookie gently into the custard. Top with half the bananas. Repeat. Make an additional decorative band using some of the banana slices.
Cover the final/top layer with whipped cream. Garnish with both reserved banana slices and reserved wafers. Loosely cover with plastic wrap and chill for 1 hour in the refrigerator before serving.