Why do you put the damp parchment on top of the chocolate mixture while making the Salted Chocolate Tart? Also why add olive oil to the filling?
Nigella's Salted Chocolate Tart (from SIMPLY NIGELLA) has a chocolate cookie crumb crust and a rich chocolate custard (pudding) filling. The filling is made and left to cool slightly before it is poured into the tart. If the filling were left to cool uncovered then the surface dries a little and a "skin" forms on the top. When the filling is poured into the tart crust this skin would break into pieces and mix in with the rest of the filling. This would mean that the filling would have small pieces of skin in it, instead of being completely smooth and silky. If you put a piece of wet baking parchment (parchment paper) onto the surface of the filling while it cools then it stops the surface from drying out and forming a skin.
The extra virgin olive oil acts as a flavour enhancer to the chocolate and also gives the tart filling a slightly richer feel and slighty softer texture. Olive oil goes surprisingly well with chocolate and you may like to try Nigella's Chocolate Olive Oil Cake as well.