In a heavy-bottomed saucepan, whisk together the milk, sugar, and cornstarch until no lumps remain.
Add the egg yolks and whisk until well incorporated.
Place the saucepan over medium heat. Cook, stirring constantly, until the custard thickens and comes to a gentle boil.
Remove from heat and stir in the butter and vanilla extract until smooth.
Let the custard cool to room temperature slightly.
In a large bowl, arrange a layer of vanilla wafers in the bottom of a serving dish.
Top the wafers with banana slices, then pour a portion of the custard over them.
Repeat layering: wafer, banana, custard, ensuring even distribution.
Finish with a top layer of wafers and a generous pour of custard. Chill in the refrigerator for at least 2 hours before serving.
Before serving, garnish with additional bananas, if desired.