In a stand mixer or using a hand mixer, whip the cold heavy cream until stiff peaks form.
In a separate bowl, whisk together the sweetened condensed milk and hazelnut butter until smooth and creamy.
Gently fold the hazelnut mixture into the whipped cream using a spatula, working from the bottom up to keep the mixture light and airy.
Line a loaf tin with cling film or parchment paper. Pour half of the hazelnut gelato mixture into the tin and smooth the top. Cover and place in the freezer for 1 hour and 30 minutes. Store the remaining gelato mixture in the fridge, for later use.
Melt the dark chocolate and set it aside to cool completely.
Remove the tin from the freezer and pour in half of the melted chocolate, spreading it evenly over the gelato base. Sprinkle half of the chopped hazelnuts on top.
Pour in the remaining gelato mixture, smooth the top, cover the tin again, and return it to the freezer for 1 hour and 30 minutes.
Place the digestive cookies in a food processor with 2-3 tablespoons of condensed milk and blend until finely crumbled.
Take the tin out of the freezer and spread the cookie mixture evenly over the gelato, pressing it down gently. Cover and return to the freezer for at least 4 hours, or until fully set.
Before serving, let the gelato cake sit at room temperature for about 5-10 minutes.
Place a large serving board or platter on top of the loaf tin, then carefully flip the tin over so the gelato cake slides out onto the board. Peel off the cling film, drizzle with the remaining melted chocolate, sprinkle over the chopped hazelnuts, and serve straight away.