Mix all the dough ingredients and knead until smooth. Halve dough and form each half into a ball. Wrap them in cling film. Leave in the fridge for 1 hour.
Preheat oven to 180°C/355°F Roll out each dough between 2 pieces of lining paper until you get a circle of 11 inches. Shape the circle with your hands, so that you will get a smooth edge. There is no need to get a perfect circle. Bake 15 minutes or until cooked and golden. Let cool.
Put milk into a pan. Slice vanilla bean lengthwise. Scratch out black stuff inside vanilla bean with the blade of a knife and add it to the milk together with the sliced vanilla bean. Bring to a boil. Let cool.
Using an electric mixer, mix yolks and sugar for 2 minutes. Add flour and mix until well combined.
Remove vanilla bean from milk. Add yolk mixture.
Stirring all the time, cook mixture on medium-low heat until thickened. The mixture mustn't boil.
Let custard cool, stirring frequently to avoid building a skin on top.
Now you've got two variations. The original recipe says to spread 1 biscuit with the cooled filling, to top it with the remaining biscuit, to dust it with the icing sugar and to sprinkle it with the toasted slivered almonds. As I feared that the slivered almonds would fall off that way when you cut it, I reserved a small amount of the filling and spread it on top of the second biscuit, topped it then with the almonds and finally dusted the foccacia di mandorle with icing sugar.