Prepare the coffee and let it cool completely.
In a large mixing bowl, beat the mascarpone cheese with the powdered sugar until smooth.
Pour in the cold heavy whipping cream and continue beating until you get a smooth, thick filling. Do not overbeat. The filling should be creamy and hold softly, not become completely stiff.
Divide the filling into two equal parts.
Dip each cocoa biscuit briefly into the cooled coffee and arrange the first layer in the bottom of your pan.
Spread the first half of the filling evenly over the biscuit layer using a spoon or spatula.
Dip more biscuits in the coffee and arrange a second layer over the filling.
Spread the second half of the filling evenly on top.
Add a third layer of coffee-dipped biscuits on top.
Refrigerate the cake for 1 hour.
Place the Nutella and coconut oil in a heatproof bowl and melt them gently over a double boiler, stirring until smooth.
Let the topping cool slightly, then pour it over the chilled cake and spread evenly.
Return the cake to the refrigerator for 2 more hours, or until fully set.
Slice and serve cold.