Game plan: The unfrozen ice cream base can be made up to 2 days in advance. It needs 3 to 4 hours to harden in the freezer after it’s been churned—that is, unless you want soft serve.
2 cupsheavy cream
1 cup whole milk
2/3 cup granulated sugar
4 large egg yolks
2 teaspoonsground cinnamon
1 teaspoonvanilla extract
Prepare an ice water bath by filling a large bowl halfway with ice and water; set aside. Set a fine-mesh strainer over a large heatproof bowl and set aside.
Combine cream, milk, and sugar in a medium saucepan and bring to a simmer over medium heat, stirring occasionally until sugar has dissolved, about 10 minutes. Remove from heat.
Meanwhile, whisk egg yolks in a large bowl until smooth. Slowly pour about 1 cup of the cream mixture into the yolks, whisking constantly so the warm cream doesn’t scramble the eggs. Pour the cream-egg mixture back into the saucepan.
Cook over low heat, stirring constantly with a wooden spoon, until the mixture thickens slightly and coats the spoon, about 5 minutes. (When you draw your finger across the spoon, it should make a mark through the mixture, which should not run back in on itself.)
Remove from heat and strain through the prepared fine-mesh strainer. Whisk cinnamon into the ice cream base until incorporated, then set the bowl over the ice bath to cool to room temperature, about 15 to 20 minutes.
Remove the ice cream base from the ice bath, cover, and place in the refrigerator to chill completely, at least 3 hours or overnight. Freeze in an ice cream maker according to the manufacturer’s instructions. The ice cream will keep in an airtight container in the freezer for up to 1 week.