Mattentaart is a round “cheesecake” made inside a puff pastry.
Cheesecake is not an entirely accurate description; “curd cake” is more accurate, though the texture is similar to that of a cheesecake.
The pastry is light, tender, and crunchy. The curd cake part of it is made from egg, either ground almond or almond extract, and milk and buttermilk that have been allowed to form curds. The buttermilk gives it a tang.
It is made in the Dutch speaking part of Belgium, in the south of the province of East-Flanders.
To be sold as Mattentaart, it can only be made in the municipality of Geraardsbergen (Grammont in French) or the nearby village of Lierde, and has to be made using the recognized recipe, using dairy from the local region.
The authorized production area of Geraardsbergen includes the towns of Goeferdinge, Grimminge, Idegem, Moerbeke, Nederboelare, Nieuwenhove, Onkerzele, Ophasselt, Overboelare, Schendelbeke, Smeerebbe-Vloerzegem, Viane, Waarbeke, Zandbergen and Zarlardinge.
Here is a video showing the production of Mattentaart. It’s in Dutch, but the visuals are self-explanatory. http://www.een.be/programmas/dagelijkse-kost/meester-mattentaart (één TV. Belgium. Link valid as of October 2014.)
You need to do the first part of the recipe 12 hours at least before you mean to bake this.
1 litre milk
250 ml buttermilk
100 grams sugar
1 teaspoon almond extract
2 sheets ready-made puff pastry
In a saucepan, bring the milk to a boil, then add the buttermilk and stir well. The mixture will curdle; it’s supposed to. Line something that you can prop over a bowl or pot with a double layer of cheesecloth, and pour the curds and whey through that. Let stand for at least 12 hours in a cool spot. You can stir the curds from time to time to aid draining.
After that time, continue with the recipe.
Begin heating oven to 200 C (400 F.)
Butter a round cake pan, bottom and sides. Line just the bottom of it with a sheet of puff pastry, cutting off loose edges. Set in fridge.
Separate the eggs. Beat whites to stiff peak; set aside. In a separate bowl, beat together the curd, egg yolk, almond extract, and sugar until smooth. Fold a small amount into the egg white, then fold all the egg white into the egg yolk and curd mixture until thoroughly but gently blended.
Take lined cake pan out of fridge. Pour the mixture into it. Top with remaining sheet of puff pastry, cutting off loose edges. Make a few slits in the top crust.
Pop in oven, bake for 10 minutes, then lower temperature to 175 C (350 F) and bake for another 15 minutes, then finally raise heat back up to 200 C (400 F) for 5 minutes. Then remove from oven, and let cool. Slice to serve.
The oldest known recipe resembling Mattentaart appears in “Een notabel boexcken van cokeryen” (A Notable Book of Cookery) 1510 by Thomas van der Noot. The recipe suggests a fast day version made with almond milk, and it comes out more like a porridge. A non-fast day version is also given, using eggs and milk.
A dish somewhat similar to Mattentaart, called “Melktert”, is made in South Africa. Some people credit the origin of both dishes to the same recipe in the van der Noot book mentioned above. 
The cake as it is known today started to emerge in the late 1700s, early 1800s.
In 1978, the Conferie de la Tarte Matton association was formed.
15 February 2007 the dish received its European PGI certification.
Literature & Lore
Geraardsbergse Mattentaart Day is celebrated in August each year, since 1979.
In Dutch, it is called “Mattentaart”; in French, “tarte aux matons.” “Matten” means “curds.”
 Claassens, Hester Wilhelmina. Die geskiedenis van Boerekos 1652-1806. Doctoral Thesis, Historical and Heritage Studies Department, University of Pretoria. 10 September 2003.