Whisk the egg yolks in a small pot until smooth, then whisk in the milk until completely combined.
Heat custard on very low heat, stirring constantly.
As soon as the custard starts to thicken, remove from heat and continue stirring.
If custard isn't quite thick enough, return to heat briefly then remove again, stirring constantly. Repeat until desired thickness is reached, but do not overcook the egg or it will go lumpy.
Add in mashed banana and mix well.
Allow to cool for a minute or two before serving.