Method:

Preheat your oven to 180°C (350°F). Grease and line your springform pan with parchment paper.

Melt the chocolate: In a heatproof bowl, melt the dark chocolate either over a double boiler or in the microwave, stirring until smooth.

Toast and grind the hazelnuts: Toast the hazelnuts in a pan over medium heat until fragrant, then grind them to a fine powder in a food processor.

Prepare the batter: In a mixing bowl, cream the softened butter and sugar together until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Combine the dry ingredients: In another bowl, sift together the flour, ground hazelnuts, baking powder, and salt.

Mix the batter: Gradually fold the dry ingredients into the wet mixture, then add the melted chocolate and mix until smooth.

Bake: Pour the batter into the prepared pan and bake for 35 minutes or until a toothpick inserted into the center comes out clean. Let the cake cool completely.

Decorate: If desired, spread a layer of ganache over the cooled cake and top with extra toasted hazelnuts.

Variations:

Gluten-Free: Use a gluten-free flour blend instead of all-purpose flour for a gluten-free version.

Dairy-Free: Swap the butter with a non-dairy butter or coconut oil, and use a dairy-free chocolate.

For Extra Flavor: Add a splash of hazelnut liqueur like Frangelico to the batter for an added depth of flavor.

Tips:

Make sure the cake is completely cooled before removing it from the springform pan to prevent cracking.

If you want the cake to be extra moist, you can drizzle it with a bit of espresso or coffee syrup.

You can make the cake ahead of time and store it in an airtight container for up to 3 days.