Crush the biscuits into fine crumbs.
Mix the biscuit crumbs with the melted butter.
Add the milk gradually until the mixture holds together when pressed and does not look dry.
Press the mixture firmly into a 25 cm pan with the back of a spoon to form an even base.
Place the base in the refrigerator while you prepare the filling.
In a mixing bowl, briefly beat the mascarpone cheese with the powdered sugar until smooth.
Add the cold heavy cream and beat until you get a smooth, thick filling. Do not overwhip.
Divide the filling into two equal parts.
Melt the Nutella gently over a double boiler and let it cool slightly.
Spread the first half of the filling evenly over the chilled base.
Place the cake in the freezer for 5–10 minutes so the layer firms up slightly.
Remove the cake from the freezer and pour the melted Nutella over the filling.
Spread the Nutella gently with a spoon into an even layer.
Return the cake to the freezer for 10 minutes so the Nutella can set.
Remove the cake and spread the second half of the filling on top.
Refrigerate the cake for 1 hour.
Place the finely chopped dark chocolate in a heatproof bowl.
Heat the heavy cream just until it comes to a boil.
Pour the hot cream over the chopped chocolate and let it sit briefly, then stir until smooth.
Let the chocolate topping cool for 2–3 minutes.
Pour the topping over the cake and spread evenly.
Return the cake to the refrigerator for 2 hours.
Slice and serve.