To make candied hazelnuts, add sugar and water to a non-stick frying pan and heat over medium low heat until the sugar dissolves. After the sugar has dissolved, increase the heat to medium and continue cooking until the mixture becomes a caramel color. This should take about 5 minutes. Stir in the hazelnuts and remove from heat. Pour the mixture over parchment paper or a silicone mat. Spread into a layer and let cool completely. Once the mixture is completely cool, use a sharp knife or a meat mallet to break up and slightly crush the candied hazelnuts. This step can be made up to 2 days ahead of time. Store the candied hazelnuts in an airtight container at room temperature until ready to use.

To make the mascarpone cream, begin by gently cooking the egg yolks. Over a double boiler, whisk the egg yolks and sugar together for 5 minutes, until pale and creamy.

In the bowl of a stand mixer fitted with the whisk attachment or in a large mixing bowl using a hand mixer, lightly beat the mascarpone cheese. Add the cooked egg yolk/sugar mixture to the mascarpone and beat until combined. Add the heavy cream and beat on medium-high speed for 5 minutes, or until the mixture thickens.

In a clean mixing bowl fitted with the whisk attachment, beat the egg whites and salt together until foamy. Increase the speed and beat until stiff peaks form. Fold the egg white mixture into the mascarpone cream mixture.

 In a shallow bowl, whisk together the coffee and rum. Dip one ladyfinger at a time, flipping it so both sides get soaked. Arrange a layer of soaked ladyfingers into the bottom of a deep 10-inch round pan or a 9x13-inch baking dish. To fill any empty spaces, cut the ladyfingers accordingly.

Spread half of the mascarpone cream on top of the ladyfingers. Sprinkle a layer of crushed candied hazelnuts on top of the cream.

Dip and layer the remaining ladyfingers, top with remaining cream. Cover with plastic wrap and chill for at least 4 hours or overnight.

 After chilling, dust the top layer of cream generously with a layer of unsweetened cocoa powder using a sifter. Top with more crushed candied hazelnuts and coffee beans, if desired.