Chop 140 g dark chocolate and place it in a heatproof bowl with 100 g Nutella.

Heat 200 ml heavy cream on the stovetop until small bubbles form around the edge; do not boil.

Pour the hot 200 ml cream over the dark chocolate and Nutella. Let stand for 3 minutes.

Stir the chocolate mixture until smooth, then mix in 30 g unsalted butter.

Combine 9 g instant coffee and 3 tbsp granulated sugar in a second heatproof bowl.

Pour 375 ml hot water into the coffee mixture and stir until dissolved.

Stir 60 ml cognac into the coffee mixture. Let the dipping liquid cool to room temperature.

Whip 500 ml heavy cream with 100 g icing sugar in the stand mixer until soft peaks form.

Mix 500 g mascarpone cheese with the remaining 60 ml cognac and 8 g vanilla sugar just until smooth.

Fold the whipped cream into the mascarpone mixture in batches until no streaks remain.

Briefly dip 12 Savoiardi biscuits in the cooled coffee mixture and arrange them in the baking dish.

Spread half of the chocolate layer over the biscuits, then cover with half of the mascarpone cream.

Dip the remaining 12 Savoiardi biscuits in the coffee mixture and arrange them over the cream.

Spread the remaining chocolate layer over the biscuits, then finish with the remaining mascarpone cream.

Cover the tiramisu and refrigerate for at least 120 minutes.

Dust the chilled tiramisu with 1 tbsp cocoa powder immediately before serving.

Before serving, sprinkle with cocoa powder.