Preheat the oven to 180 degrees Celsius /350 Fahrenheit.
To make the sponge, crack the eggs and add them to a bowl.
Add the sugar and beat them until fluffy and the colour becomes a pale yellow.
In a separate bowl, mix together the flour, salt, cocoa powder and baking powder and add the dry ingredients to the egg mixture one tablespoon at a time, mixing well with the spatula.
Once the dry ingredients are fully incorporated, add the vanilla extract, mix again lightly.
Butter and flour a square baking tray of 26/36 cm/10x14 and pour the mixture in.
Bake in the preheated oven for about 20 minutes or until risen and firm to touch.
Set aside to cool completely.
To make the filling, make sure that you remove the butter from the fridge at least one hour before, so that it can be really soft.
Place the butter in a large bowl and add the sugar, cocoa powder, nutella and vanilla extract.
Use an electric mixer to beat everything well until smooth and creamy.
To make the syrup, place the water and sugar into a saucepan and bring it to a boil, stirring until the sugar has dissolved completely.
Add the coffee and the vanilla extract, remove from the boil and allow it to cool completely.
To make the glaze, sift the icing sugar and cocoa powder, add the oil and boiling water, and mix until smooth. Leave to cool completely.
Next, cut the sponge into 2 parts horizontally, and place one part onto the baking tray.
Use a brush to brush the cake with the syrup, we want it quite moist, but not too soggy.
Next, spread the chocolate filling over the entire cake, place the second part of the sponge on top of the cream and brush it again with the syrup.
Top the cake with the chocolate glaze, the refrigerate for at least 4 hours, ideally overnight.
Use a sharp knife to cut the cake into squares, then decorate with whipped cream. Serve chilled.