In a medium saucepan, bring milk and cream to a boil over medium-high heat.
Meanwhile, in a medium bowl, whisk egg yolks and sugar until smooth. Gradually whisk cream mixture into egg-yolk mixture. Return mixture to saucepan and cook over medium-low heat, stirring constantly, until it thickens and coats the back of a spoon (do not boil or it will curdle).
Remove saucepan from heat; whisk in sweet potato puree. Strain custard through a sieve into large bowl. Refrigerate custard until well chilled, at least 2 hours, but no more than overnight.
Freeze in ice cream maker as directed by manufacturer.