This delicious recipe comes from David Lebovitz, with a few minor alterations (http://www.davidlebovitz.com/sugarfree-choco/).
Start this recipe the day before you want to serve it, so the custard can chill overnight in the refrigerator.
In a small saucepan, warm the agave syrup with the unsweetened chocolate over the lowest heat possible, stirring constantly, until the chocolate is melted. Remove from heat and transfer 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 moderate 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 scrape the mixture into the bowl with the chocolate-agave mixture. Stir them together, then set a mesh strainer over the top.
Add the remaining half-and-half to the saucepan with a pinch of salt, turn the heat to low, and when warm, slowly pour the warm half-and-half into the egg yolks whisking constantly, then pour the warmed yolks back into the saucepan.
Cook, stirring constantly over low heat, until the mixture becomes steamy and thickens. If using an instant-read thermometer, it should read about 170F degrees. (76C). (NOTE: This is where both Stefin and Andrea ended up with scrambled eggs, so use low heat and watch carefully.)
Pour the egg mixture through the strainer into the chocolate mixture.
Stir, then let cool a few minutes until slightly warm. Once it’s cooled down, throw the mixture in a blender for ten seconds until it’s smooth and velvety. (NOTE: Andrea skipped this step and things turned out fine.)
Chill the custard thoroughly in the refrigerator overnight, then freeze in your ice cream maker according to the manufacturer’s instructions.