Cut each ear of corn in half crosswise. Cut corn kernels from cobs, reserving corn cobs. (You should have about 2½ cups of cut corn.) Place cut corn and cobs in a large saucepan.
Add cream, milk, ½ cup sugar, and rosemary sprigs to corn mixture. Bring to a low boil over medium-high heat, stirring frequently. Remove from heat. Let stand at room temperature for 1 hour to allow flavors to infuse. Discard corn cobs and rosemary sprigs.
Transfer cream mixture to container of a blender. Cover and process until smooth. Return corn-cream mixture to saucepan. Bring just to a simmer over medium heat, stirring frequently. In a small bowl, whisk together egg yolks, salt, and remaining ¼ cup sugar. Add 1 cup of hot corn mixture to yolks, stirring constantly. Add yolk mixture back to saucepan, stirring constantly, and cook over medium-low heat until mixture thickens enough to coat the back of a metal spoon, 10 to 12 minutes.
Strain custard through a fine-mesh sieve into a medium bowl, discarding solids. Whisk sour cream into strained custard. Let custard cool in an ice bath for 30 minutes. Cover and chill for at least 4 hours or overnight.
Freeze corn mixture in an ice-cream maker according to manufacturer’s directions. Transfer ice cream to freezer containers, allowing space for expansion. Freeze 4 hours or until firm. Garnish with rosemary sprigs, if desired.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/sweet-corn-rosemary-ice-cream/