If your food processor bowl holds less than 11 cups, puree half the pudding with the roasted bananas and lemon juice in step 3, transfer it to a large bowl, and whisk in the rest of the pudding.
To roast the bananas:
Adjust oven rack to upper-middle position and heat oven to 325 degrees. Place 3 unpeeled bananas on baking sheet and bake until skins are completely black, about 20 minutes. Let cool 5 minutes.
To make the pudding:
Meanwhile, whisk 1/2 cup sugar, egg yolks and cornstarch in medium bowl until smooth. Bring half-and-half, remaining sugar, and salt to simmer over medium heat in large saucepan. Whisk 1/2 cup simmering half-and-half mixture into egg yolk mixture. Slowly whisk tempered yolk mixture into saucepan. Cook, whisking constantly, until mixture is thick and large bubbles appear at surface, about 2 minutes. Remove from heat and stir in butter and vanilla.
Process the pudding by transferring it to food processor. Add warm peeled roasted bananas and 2 tablespoons lemon juice and process until smooth. Scrape into large bowl and place plastic wrap directly on surface of pudding. Refrigerate until slightly cool, about 45 minutes.
To assemble and chill:
Cut remaining bananas into 1/4-inch slices and toss in bowl with remaining lemon juice. Spoon one-quarter of pudding into 3-quart trifle dish and top with layer of cookies, layer of sliced bananas, and another layer of cookies. Repeat twice, ending with pudding. Place plastic wrap directly on surface of pudding and refrigerate until wafers have softened, at least 8 hours or up to 2 days.
Next, prepare to top and serve. With electric mixer on medium speed, beat cream, sugar and vanilla until stiff peaks form, about 2 minutes. (Whipped cream can be refrigerated for 4 hours.) Top banana pudding with whipped cream. Serve.
To make toasted-coconut banana pudding:
Prepare banana pudding, replacing 2 cups half-and-half with 1 (16-ounce) can coconut milk in step 2. Sprinkle 1/4 cup toasted sweetened shredded coconut over whipped-cream-topped pudding before serving.
To make peanut-y banana pudding:
Prepare banana pudding. In step 4, sandwich 2 vanilla wafers around 1 banana slice and 1/2 teaspoon creamy peanut butter (you'll need 1/2 cup total). Assemble by alternating layers of pudding and cookie-banana sandwiches, ending with pudding. Sprinkle 1/4 cup chopped salted dry-roasted peanuts over whipped-cream-topped pudding before serving.
Test Kitchen technique:
We put the banana back in banana pudding. Most recipes for banana pudding actually use vanilla pudding. We put true banana flavor where it belongs — in the pudding itself — by roasting slightly under-ripe bananas to make them richer and sweeter, and to break down their fibers.