Combine nutella and chocolate, broken into small pieces, in a large microwave-safe bowl. Microwave, stopping and stirring every 30 seconds or so, until mixture is melted and smooth. Let cool slightly.
Combine the egg yolks and half the sugar in a medium-size bowl. Whisk vigorously until the yolks thicken and become lemon-colored, 3 to 4 minutes.
In a heavy saucepan, combine the milk and cream. Sprinkle about half the sugar into the saucepan and bring the mixture to a simmer over medium heat. Remove from heat just when steam starts to form.
To combine the egg and milk mixtures, slowly pour half the milk into the yolks while whisking constantly to temper it. Continue whisking in the rest of the milk. Pour mixture back into the saucepan and reduce heat to medium-low. Whisk constantly until the custard is thick enough to coat the back of a spoon (175 F on a digital thermometer), 10-15 minutes. Do not let the custard boil or it will curdle!
Pour the cooked custard over the chocolate mixture. Whisk until the chocolate is fully melted (I whisk a good 5 minutes for the nutella to fully incorporate).
If cooling by ice bath, fill a large bowl (capable of holding the bowl with the custard) with lots of ice and very cold water. Set the bowl with the custard on top and allow it cool, stirring occasionally, until the mixture reads 40-55 F or is very cold to the touch. This takes me about 30 minutes. Or, if you prefer, refrigerate for several hours or overnight until very cold.
Add the optional 1 Tbs of vodka. This will ensure that the ice cream will freeze with the smoothest texture and has no noticeable effect on the final taste.
Pour the custard into an ice cream machine and freeze according to the manufacturer's directions.