Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

In a bowl, whisk together the flour, cocoa powder, salt, and baking powder.

In another large bowl, beat the eggs and sugar until thick and pale. This should take about 5 minutes.

Stir in the vanilla extract.

Gently fold the dry ingredients into the egg mixture until just combined.

Spread the batter evenly onto the prepared baking sheet.

Bake for 10-12 minutes, or until the cake springs back when touched lightly.

While the cake is still warm, turn it out onto a clean kitchen towel dusted with powdered sugar.

Carefully peel off the parchment paper.

Roll the cake with the towel from the short end, forming a log. Allow it to cool completely.

Once the cake is cooled, unroll it gently.

Spread a layer of hazelnut spread over the cake.

Sprinkle with chopped hazelnuts.

In a bowl, beat the heavy cream until stiff peaks form.

Spread the whipped cream over the hazelnut filling.

Reroll the cake without the towel tightly and place it seam-side down on a serving platter.

Melt the dark chocolate and butter together until smooth.

Allow it to cool slightly before drizzling over the cake.

Refrigerate the cake for at least 1 hour before slicing.