Prepare an ice-water bath. Stir together sugar and 1 cup water in a medium saucepan. Bring to a boil, stirring until sugar is dissolved. Remove from heat; pour into a metal bowl set in ice-water bath. Let cool completely, stirring occasionally.
Whisk in buttermilk, cream, corn syrup, salt, and vanilla seeds (reserve bean for another use). Freeze in an ice cream maker according to manufacturer's instructions. Transfer to an airtight container and freeze at least 1 hour and up to 3 days.
Scoop ice cream into serving dishes and top with a spoonful of applesauce. Garnish with apple slices; serve immediately.