Meanwhile, in a medium bowl whip the egg yolks with the remaining sugar using an electric mixer until the eggs have become thick and pale yellow, about 4 minutes. Pour 100ml of the warm milk and cream mixture into the egg mixture and stir. Add this mixture back into the saucepan. Cook over very low heat, stirring constantly, until the mixture becomes thick enough to coat the back of a wooden spoon, about 7 to 10 minutes.
Place a strainer over a medium bowl and pour the warm custard mixture through the strainer. Stir in the vanilla and hazelnut spread until it dissolves. Chill mixture completely before pouring into an ice cream maker and follow manufacturer's instructions to freeze. To serve, scoop gelato into serving bowls and top with hazelnuts.
Makes about 950ml gelato