Preheat oven to 170°C (340°F).
In a bowl, whisk egg + sugar until smooth.
Add milk, melted butter, vanilla, salt, and whisk well.
Whisk in flour (if using) until no lumps remain.
Stir in any optional fruit.
Pour mixture into a greased ramekin.
Place ramekin on a baking tray.
(Optional: add hot water to the tray halfway up the ramekin for extra-smooth custard.)
Bake 30–35 minutes until puffed, golden on top, and just set in the center.
Cool 5–10 minutes before serving.
