2 1/2 cups half & half
5 egg yolks
3/4 cup sugar
6 ounces gianduja
2 tablespoons Frangelico liqueur
Make the custard
Heat the half & half in a saucepan over medium-low heat, stirring occasionally until simmering and whisps of steam begin to appear (about 140 °F).
While the half & half is heating, beat the egg yolks and sugar until blended. The yolks will turn a light yellow and fluff up just a little. Use a spatula to scrape the bottom of the bowl during mixing.
With the mixer on low, slowly add the hot half & half to the egg yolk and sugar mixture. Mix until blended and then return this mixture to the saucepan. Heat the custard mixture over medium heat, stirring constantly, until mixture thickens and a finger leaves a track when dragged across the back of a spoon (about 165 °F).
Incorporate the gianduja
Pour the custard mixture through a chinois or sieve to remove any egg bits and set aside. Chop the gianduja into small pieces (to speed up melting) and place in a double boiler over very hot (not simmering) water. Stir the gianduja to speed up melting. Add about 1/4 cup of the hot custard to the melted gianduja and stir until incorporated. Add another 1/4 cup of custard and repeat the process, increasing the amount of custard you add until blended. If you pour it in all at once, there is a good chance the gianduja will turn grainy.
Add liqueur and freeze
Place bowl with gelato base into an ice bath or refrigerate until mixture is chilled to about 45 °F. Add the 2 tablespoons of Frangelico and stir until blended. Freeze according to your ice cream/gelato maker's instructions and transfer to a container. Let ripen in freezer for several hours before serving.