Put the half-and-half and orange zest in a small saucepan and bring to a simmer over high heat. Reduce the heat to low, add the chocolate, and let sit for 1 minute. Whisk until smooth, melted, and well combined.
Whisk together the egg yolks, sugar, salt and cornstarch in a medium bowl until pale and fluffy, about 2 minutes. While whisking continuously, slowly add the hot chocolate mixture to temper the eggs. Return the mixture to the saucepan and cook, over medium heat, whisking continuously, until thickened, about 5 minutes. Remove from heat and stir in the vanilla and Frangelico.
Set a strainer over a medium bowl. Strain the chocolate custard into the bowl, then divide the custard between eight 3-ounce ramekins. Cover the tops with plastic wrap pressed tightly against the surface of the custard to prevent a skin from forming. Refrigerate until set, at least 2 hours and up to 24 hours.
While the custard is chilling, line a half sheet pan with a silpat or a sheet of parchment paper sprayed liberally with nonstick spray. Combine the sugar and ¼ cup of water in a medium saucepan set over low heat. Cook, stirring once, until the sugar is dissolved, then increase the heat and cook until it turns a deep amber color, about 7 minutes. Add the nuts and swirl the pan to evenly coat the nuts in the caramel. Carefully pour the mixture onto the prepared pan in an even layer. Let harden at room temperature, about 15 minutes. Break into large pieces and top each custard with the brittle.
