Brew strong coffee and let it cool to room temperature. Set aside.
Spread a thin layer of Nutella on one Savoiardi, then press a second Savoiardi on top to make a sandwich. Repeat until you’ve used all Savoiardi.
Arrange the Savoiardi sandwiches in a snug single layer in a 20×20 cm pan.
Slowly pour the cooled coffee evenly over the Savoiardi so they soak it up. Don’t add too much at once or the biscuits may fall apart.
In a mixing bowl, beat the mascarpone with powdered sugar until smooth.
Add the cold heavy cream and whip until thick and creamy. The filling should be smooth and spreadable, not completely stiff.
Spread the filling evenly over the soaked Savoiardi layer and smooth the top.
Refrigerate for at least 2 hours.
Right before slicing, dust the top with cocoa powder (about 3–4 tablespoons). Slice and serve.