In a small mixing bowl, whisk together 1/4 cup milk with the cornstarch until smooth, set aside.
In a medium saucepan, combine remaining 1 3/4 cup milk, the cream, sugar, corn syrup, vanilla bean seeds and vanilla bean pod. Bring mixture just to a boil over medium-high heat, stirring frequently to dissolve sugar.
Once it reaches a boil, whisk cornstarch mixture once more, and while whisking milk mixture in saucepan, slowly pour cornstarch mixture into saucepan. Reduce heat slightly, while stirring constantly, allow mixture to bubble and cook about 1 - 2 minutes longer until has thickened and it coats the back of a wooden spoon.
Remove from heat and whisk in sour cream. Pour into a heat proof bowl. Cover mixture with wax paper, pressing directly against the surface of the mixture to prevent a skin from forming, and refrigerate until chilled through* (after which I also recommend freezing for 30 minutes, then stirring - yes before processing it in the ice cream maker, unless you have a really professional grade one. It gives the ice cream a better texture and gives the ice cream maker a head start).
Process mixture in an ice cream maker according to manufactures instructions. Transfer mixture to an airtight container and freeze until firm (if your freezer is as cold as mine, let it rest at room temp several minutes before serving for optimal results. It should be semi-soft).
Notes
*If you are looking for a speed chill method before processing, you can set the bowl with ice cream mixture in another large bowl, (I use my large dutch oven), then place ice around bowl and pour cold water over ice, then place in freezer and stir about every 10 minutes until mixture is very cold (returning wax paper over top to cover after stirring).