1. Set a large, heat-tolerant bowl and a strainer close at hand.
2. Whisk the egg yolks together in a second spacious, heat-tolerant bowl and set them aside.
3. Mix the cream, milk, and sugar together in a heavy saucepan.
4. Place the saucepan over a medium heat and stir until the sugar dissolves.
5. Temper the yolks by slowly adding one cup of the hot cream mixture while whisking vigorously.
6. Reduce the heat to medium-low and whisk the tempered yolks into the saucepan, creating a custard. Stir constantly until the custard thickens slightly and coats the back of a wooden spoon. Immediately strain it into the clean bowl.
NOTE: Stir carefully and do not let the custard boil, or the yolks will curdle.
7. Place the espresso powder into a small, heat resistant mixing bowl and pour the boiling water over it to dissolve the crystals.
8. Mix the espresso into the custard and cool to room temperature (approximately one hour).
9. Stir in the vanilla extract and refrigerate the mixture for 4–6 hours, or overnight.
10. Place the cold custard in the canister of an ice cream maker and process according to the manufacturer’s instructions.
11. Transfer the ice cream to a container, cover, and freeze for at least 4 hours to firm up.
NOTE: For best flavor, always remove ice cream from the freezer 20 minutes before serving, or place it in the microwave for 20 seconds.