Mix together the ingredients for the custard in a saucepan and cook until the mixture thickens.
Set aside to cool.
Combine all the ingredients for the dough in a large bowl and knead until smooth and as soft as an earlobe.
Cover with a damp cloth and leave to rest for 15 minutes.
Roll out the dough with a rolling pin into an oval shape and spread the custard over it.
Set aside to rise for 50-60 minutes at room temperature.
Brush the dough with the egg yolk and bake at 190-200C for 15-20 minutes in a preheated oven.
