In a medium heavy-bottomed saucepan, combine the milk and cream. Heat over medium until steaming and small bubbles form around the edges, about 5 minutes. Do not boil.

While the dairy heats, whisk the egg yolks, sugar, cornstarch, and salt in a large bowl until pale yellow and slightly thickened, about 2 minutes.

Slowly drizzle about 1 cup of the hot milk mixture into the yolks, whisking constantly to prevent scrambling. Pour the tempered yolks back into the saucepan with the remaining milk.

Cook over medium heat, whisking constantly, until the custard bubbles and thickens enough to coat the back of a spoon, 4 to 6 minutes. It should hold a line when you run your finger through it.

Remove from heat. Whisk in the vanilla and cold butter until fully melted and smooth. Strain through a fine-mesh sieve into a clean bowl. Press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate until cold, at least 1.5 hours.

Slice the bananas into 1/4-inch rounds and toss with lemon juice to prevent browning.

Spoon a thin layer of custard into your serving dish. Add a layer of banana slices and Nilla wafers if using. Repeat layers, ending with custard on top. Cover and refrigerate at least 30 minutes before serving.