Preheat your oven to 177 °C (175 °C). Grease an 8-inch springform pan thoroughly and line the bottom base with a round piece of parchment paper.

In a heatproof bowl set over a pot of gently simmering water, melt the chopped dark chocolate and cubed unsalted butter together, stirring continuously until completely smooth. Remove from the heat source and allow it to cool for 5 minutes.

Whisk the granulated sugar into the melted chocolate mixture. Add the room temperature eggs one at a time, beating vigorously after each addition until fully incorporated. Gently fold in the sifted cocoa powder until a cohesive, smooth batter forms.

Pour the prepared batter evenly into the lined springform pan. Bake for 22 to 25 minutes, or until the center is set but still slightly soft to the touch. Remove from the oven and let the base cool completely inside the pan on a wire rack.

In a medium bowl, gently stir together the crisp rice cereal and the warmed hazelnut chocolate spread until every piece of cereal is completely coated. Carefully press this crisp mixture into an even layer directly over the top of the cooled chocolate cake base.

In a separate chilled large bowl, whip the heavy cream, powdered sugar, and vanilla extract together until stiff peaks form. Spread the whipped cream evenly over the crunchy cereal layer, creating a smooth or textured top surface as desired.

Place the assembled cake into the refrigerator and chill for a minimum of 2 hours to allow all the layers to set properly before slicing and serving.