In a 4-quart stainless steel saucepan or pot, combine milk and scraped vanilla bean along with its seeds or 1 teaspoon vanilla paste. Bring to a bare simmer over medium heat. Remove from heat, cover to prevent evaporation, and let steep for 30 minutes.

In a large bowl, set up an ice bath by partially filling it with a combination of cold water and ice. Set aside.

In a medium heatproof mixing bowl set on top of a dampened towel (this serves as a stable base), stir together sugar, cornstarch, and salt. Whisk in egg yolks until mixture is pale yellow, smooth, and fluffy, about 1 minute.

Uncover infused milk and remove vanilla bean. While whisking continuously, slowly pour milk into egg yolk mixture in a thin stream, until all of it has been added.

Return mixture to the same pot. Cook over medium heat, whisking constantly until pastry cream begins to thicken, about 5 minutes. Once it thickens, continue to whisk, pausing every few seconds to check for bubbles, about 1 minute. When it begins to bubble, set a timer and continue whisking for 1 minute. (This step is important to neutralize amylase, a starch-dissolving protein found in egg yolks.)

Off-heat, whisk in butter until melted and thoroughly combined. Strain pastry cream through a fine-mesh sieve set over a heatproof medium bowl. Immediately place plastic wrap directly on the surface of the cream to prevent a skin from forming. Transfer bowl to prepared ice bath to chill for 30 minutes, then refrigerate until cold, about 2 hours.

When ready to assemble banana pudding, whisk pastry cream until smooth. Using a flexible or offset spatula, cover bottom of trifle bowl with about 1 cup pastry cream. Top with a layer of Nilla Wafers. Arrange banana slices on top of cookies. Repeat until you have 4 layers; cover top layer with remaining pastry cream. Place plastic wrap directly on surface of pastry cream and refrigerate until wafers have softened and the flavors have melded, at least 8 hours and up to 24 hours. (You may have remaining Nilla Wafers; set aside to garnish, if desired.)

When ready to serve, prepare the whipped cream: In the bowl of a stand mixer fitted with a whisk attachment, whip heavy cream, confectioners sugar, and remaining ½ teaspoon kosher salt until soft peaks form. (Alternatively, in a medium bowl, whisk cream by hand or using a hand mixer until soft peaks form.)

Remove plastic wrap from pudding and top with whipped cream. Garnish with crushed Nilla Wafers, if desired. Serve immediately.