Move oven rack to upper-middle position and preheat oven to 325 degrees F. Cover baking sheet with foil and place 3 unpeeled bananas on top. Bake until skins are completely black, about 20 minutes. Let cool 5 minutes.
Meanwhile, in a medium bowl whisk together 1⁄2 cup sugar, egg yolks, and cornstarch.
In a large saucepan over medium heat bring half-and-half, remaining sugar, and salt to simmer. Whisk 1⁄2 cup simmering half-and-half mixture into egg yolk mixture.
Slowly whisk tempered yolk mixture into the saucepan. Cook, whisking constantly until mixture is thick and large bubbles appear at the surface, about 2-3 minutes.
Remove from heat and stir in butter and vanilla.
Transfer pudding to a food processor. Add warm peeled roasted bananas and 2 tablespoons lemon juice and process until smooth.
Scrape pudding into a large bowl and place plastic wrap directly on the surface of pudding. Refrigerate until slightly cool, about 60 minutes.
ASSEMBLE AND CHILL:
Cut remaining bananas into 1⁄4-inch slices and toss in a small bowl with remaining lemon juice. Spoon one-quarter of pudding into a 3-quart dish. Top with a layer of cookies, layer of sliced bananas, and another layer of cookies.
Repeat twice, ending with pudding.
Place plastic wrap directly on the surface of pudding and refrigerate until wafers have softened, at least 8 hours or up to 2 days.
TOP AND SERVE:
In a bowl of electric mixer, beat cream, sugar, and vanilla on medium speed until stiff peaks form, about 2 minutes.
Top banana pudding with whipped cream, and garnish with vanilla wafers (optional). Serve.