Preheat oven to 325°F (165°C). Spread hazelnuts on a baking sheet and roast for 10–12 minutes until golden and fragrant. Let them rest for a few minutes, then rub in a kitchen towel to remove most skins.

Add hazelnuts to a food processor with 2 tablespoons of sugar. Pulse in short bursts until finely ground but not oily—stop as soon as the texture looks sandy.

In a clean bowl, beat egg whites until soft peaks form. Gradually add the remaining sugar and continue beating until glossy. Gently fold in the ground hazelnuts, flour, salt, and softened butter using a spatula—just until no streaks remain.

Line an 8–9 inch springform or tart pan with parchment paper. Pour in the batter and smooth the top. Bake at 350°F (175°C) for 25–30 minutes, or until lightly golden around the edges and the center springs back when touched.

Let the tart cool in the pan for about 10 minutes. Run a thin knife around the edge to loosen it, then let it finish cooling on a wire rack.

In a small saucepan, warm the cream until it just begins to simmer. Pour it over the chopped chocolate in a bowl. Let sit for 1 minute, then gently whisk from the center until smooth and shiny. Let the ganache cool until thick but still spreadable or pipeable.

Spread or pipe the ganache over the cooled tart. For a glossy finish, let it set at room temperature for about 30 minutes. Decorate with caramelized hazelnuts or dust lightly with powdered sugar.

Slice and serve at room temperature with a shot of espresso or a small drizzle of warm chocolate sauce.