In a medium saucepan over medium heat, combine the butter, sugar, cornstarch, and salt. Whisk constantly for about 1 minute until the mixture is smooth and bubbly.
In a separate small bowl, lightly beat the egg yolks. Slowly pour about 1/2 cup of the hot butter mixture into the yolks while whisking continuously. Then, pour the yolk mixture back into the saucepan.
Gradually whisk in the milk in a steady stream. Continue cooking over medium heat, stirring constantly, until the mixture thickens and comes to a gentle boil (about 5-7 minutes).
Pour the hot custard through a fine-mesh strainer into a large bowl to catch any lumps. Let it cool for 10 minutes at room temperature, then cover with plastic wrap directly on the surface to prevent a skin from forming. Refrigerate for 1 hour until completely cool.
While the custard chills, prepare your serving dish (an 8×8-inch glass dish works perfectly). Spread a thin layer of custard on the bottom. Arrange a single layer of banana slices over the custard, then drizzle with a third of the peanut butter (warm it slightly for easier drizzling). Spire with crushed graham crackers.