Makes a 10x1’’/25x2.5cm deep dish pizza pan. Grease and flour the cake pan.
In a stand mixer fitted with the paddle attachment, combine flour, cocoa powder, salt, sugar, baking powder and baking soda together. Add the room temperature butter and sand for a minute or so. Add eggs and mix to combine on low. Wrap up pastry and chill a couple of hours or more.
In a small pastry bowl, beat the egg with sugar and corn starch; set aside. Meanwhile, heat milk and whisk in nescafe and boil. Temper the egg mixture with half of the hot milk; transfer mixture back into the saucepan. Bring to boil and cook for 2 minutes; whisking constantly. Turn heat off and whisk in cocoa powder, add the chopped dark chocolate or disks – allow chocolate to melt and whisk well until smooth. Blend with immersion blender to smooth out. Transfer chocolate custard onto a dish lined with plastic wrap, cover custard with plastic wrap in contact. Cool to room temperature and refrigerate completely.
Beat the chilled chocolate custard until smooth and set aside. On a floured work surface, divide chilled chocolate crust into 2 equal portions. Roll out bottom into a generous 1/4’’/6 to 7mm thick disk. Drape the pastry over the prepared pan. Using your fingers, press dough into place in pan. Fill evenly with the chocolate custard. Roll out the second disk into the same thickness and place over the pan. With the rolling pin, trim off excess dough and remove excess flour. With a pastry brush, paint cake with a light, even coat of egg wash and design strips using the back of a fork. Chill cake an hour before baking or refrigerate for up to 3 days. It can be frozen for weeks as well. Allow gateau basque to thaw overnight in the refrigerator prior baking.
Bake at 350ºF/180ºC for 35 to 40 min. Cool to room temperature and chill over night before unmolding. Warm up bottom pan to ease pan removal. The cake can be refrigerated for up to 3 days. Enjoy!