Gradually heat the milk, rum, and whole vanilla bean with the scrapings. Don’t let it boil! Turn off the heat when it reaches 180 to 185ºF (82 to 88ºC). Remove from the heat to use to make the custard.
The cornstarch thickens and sets the custard. The trickiest part is not curdling the eggs, so I use a tempering technique.
Temper to prevent curdling
Making the custard is simple but requires constant attention. Add a small amount of warm milk to the egg, sugar, and cornstarch mixture. This process allows the eggs to rise gently in temperature and mix with the hot liquid without shocking the eggs’ albumin proteins.
The custard then cooks on the stovetop, continually stirring until it thickens. It’s normal to see some small clumps. Whisk in the butter to add richness. I always strain the custard through a fine-mesh sieve when it’s still hot to achieve a smooth consistency. You can cool it slightly so it’s not steaming hot, but don’t let it reduce too much. Otherwise, the starches will become rigid and solidified, making them harder to spread.
Fill and chill
For a strong banana flavor, cut pieces into ¼-inch thick slices and layer them on the crust’s bottom and sides. Evenly spread the warm custard on top, then cover it with plastic wrap and refrigerate. The starches and butter in the filling will set after about 3 hours, or you can just let it sit overnight.
Pull the pie out when you’re ready to make the whipped cream so the butter in the crust can come to room temperature, so it’s more tender, making it much easier to slice.
