I used a 24cm x 19 cm (9½" x 7½") dish for this recipe.
Add warm coffee (125ml) and light brown sugar (1 tsp) to a small bowl (that is the right size & shape to fit a whole biscuit) and mix to combine. Set aside to cool.
Place Nutella (350g) and milk (80ml) in a medium bowl and mix until combined (adding more milk as needed) and set aside.
In a large bowl, combine cream (250ml), icing sugar (60g) and vanilla (½ tsp) and whip into soft peaks.
Fold mascarpone (500g) into cream mixture to combine.
One at a time, dip savoiardi biscuits into coffee, and then place into the base of your dish to create a single layer (cutting to fit if needed - see my notes).
Spread Nutella mixture over biscuit layer, and spread out evenly to cover.
Spoon half of the cream mixture over the Nutella layer, and spread out evenly to cover.
Repeat with another layer of coffee soaked biscuits.
Finish with remaining cream, cover and refrigerate for 4 to 5 hours - ideally overnight.
Dust the top with cocoa powder to serve.