Step 1

Stir together milk, cream, and zest in a medium saucepan. Cook over medium-low, undisturbed, just until steaming, making sure mixture doesn't bubble, about 5 minutes. Remove from heat; cover and let infuse about 10 minutes.

Step 2

Meanwhile, whisk together egg yolks, sugar, and salt in a medium bowl until smooth. Set aside until ready to use.

Step 3

Pour infused milk mixture through a fine wire-mesh strainer into a separate bowl; discard solids. Gradually whisk infused milk into yolk mixture. Wipe saucepan clean.

Step 4

Pour milk-yolk mixture back into cleaned saucepan. Cook over medium-low, stirring constantly using a wooden spoon, until mixture is thick enough to coat the back of a spoon, 6 to 10 minutes, making sure mixture doesn't bubble. Remove from heat. Pour through a fine wire-mesh strainer into a medium bowl; discard solids. Whisk in pistachio paste. Place medium bowl in a large bowl filled with ice water, and let stand, stirring occasionally, until mixture reaches room temperature, about 8 minutes. Transfer mixture to a resealable container; seal and chill at least 1 hour or, preferably, 12 hours.

Step 5

Pour chilled pistachio mixture into frozen freezer bowl of an ice cream maker; proceed according to manufacturer's instructions for soft-serve consistency.

Step 6

Meanwhile, microwave chocolate and oil in a small microwavable bowl on high until just melted, about 1 minute, stopping to stir every 15 seconds. Let cool about 5 minutes. Set aside until ready to use.

Step 7

When gelato reaches soft-serve consistency and with maker running, gradually stream in chocolate until well combined, about 1 minute. Transfer to an 8- x 4-inch loaf pan; cover with plastic wrap pressed directly on gelato surface. Freeze until firm, about 4 hours. Store gelato in an airtight container in freezer up to 1 week. To serve, transfer gelato to refrigerator until temperature reaches 16°F or gelato is easily scoopable, about 40 minutes.