Preheat the oven to 350°F (175°C).

Grease an 8-inch or 9-inch springform pan and line the bottom with parchment paper.

Beat the egg whites until soft peaks form.

Gradually add the sugar and continue beating until stiff, glossy peaks form.

In a separate bowl, combine the egg yolks, flour, and ground walnuts.

Gently fold the walnut mixture into the whipped egg whites until just combined.

Transfer the batter to the prepared pan and smooth the surface.

Bake for approximately 25 minutes or until a toothpick inserted into the center comes out clean.

Allow the sponge to cool completely.

Drizzle the freshly squeezed orange juice evenly over the cooled sponge.

Heat the milk in a saucepan over medium heat.

Add the dark chocolate and stir until completely melted.

Stir in the orange zest and orange juice.

Add the ground walnuts and mix until thick and well combined.

In a separate bowl, whisk the egg yolks and sugar until pale and smooth.

Gradually stir the egg mixture into the walnut mixture.

Add the softened butter and mix until the filling becomes smooth and creamy.

Allow the filling to cool slightly.

Spread the walnut filling evenly over the orange-soaked sponge.

Smooth the surface with a spatula.

Refrigerate for at least 2 hours or until the filling is set.

Beat the heavy whipping cream and powdered sugar until stiff peaks form.

Spread or pipe the whipped cream over the chilled cake.

Garnish with chocolate shavings, walnuts, or orange zest if desired.

Chill for an additional 30 minutes before serving.

Slice and serve chilled.