In a small saucepan over low heat, combine agave syrup with the unsweetened chocolate, stirring constantly, until the chocolate is melted. Remove from heat and transfer mixture to a large bowl; Set aside.
In a medium saucepan, add 1½ cups of the half-and-half and whisk in the cocoa powder. Cook over medium heat until the mixture begins to bubble, then simmer for 30 seconds, whisking frequently, making sure to break up any clumps of cocoa powder. Remove from heat and transfer the mixture into the bowl with the chocolate-agave mixture. Set the bowl over a large container of ice water, then set a mesh strainer over the top.
In a separate bowl, whisk the yolks together until smooth; set aside. Add the remaining half-and-half to the saucepan with a pinch of salt, turn on the heat, and when warm, slowly pour the warm half-and-half into the yolks whisking constantly. Then slowly whisk the tempered yolk mixture back into saucepan. Continue to cook over medium heat until the mixture becomes steamy and thickens (170 F degrees).
Pour the mixture through the strainer into the chocolate mixture set in the ice bath and let cool, stirring occasionally, for about 10 minutes. Remove the custard mixture from the ice bath, and place in a blender. Blend for 10 seconds until smooth and velvety. Transfer to a bowl, cover tightly with plastic wrap and refrigerate until very cold (40 degrees), about 3 hours.
Start ice cream maker and add the mixture. Mix until ice cream has the consistency of soft serve ice cream, about 20 to 25 minutes.
Remove from ice cream machine canister and transfer to an airtight container. Freeze until firm, at least 3 hours.