Place the sugar in the bowl of a stand mixer.
Use a paring knife to split the vanilla bean lengthwise, then turn the knife over and use the dull edge to scrape the seeds into the sugar. (Save the pod for another use - like making vanilla sugar.)
Blend on low speed until the seeds are evenly dispersed.
Add the butter and beat on medium-high until the mixture is very light—almost white in color, 4 to 5 minutes.
Beat the eggs with the vanilla in a small bowl.
With the mixer running on medium speed, add the eggs to the butter mixture about 1 tablespoon at a time, allowing each addition to completely blend in before adding the next.
Sift the cake flour, baking powder, and salt into a medium bowl and whisk together.
With the mixer on the lowest speed, alternate between adding the flour mixture and sour cream, ending with the flour mixture.
Fill cupcake liners 3/4 full.
Bake at 350 F. For mini cupcakes, bake for 12 min. For regular cupcakes, bake for 20 minutes.