In a small bowl, combine sugar and seeds from the vanilla bean.
Using the back of a spoon, move around the bowl and apply pressure to break up any clumps of seeds and to better infuse the vanilla flavor into the sugar. Set aside.
In a medium-sized mixing bowl or bowl of a stand mixer, mix together cake flour, baking powder, baking soda, and salt.
Add the vanilla bean sugar and mix until well combined.
Add butter and mix on medium-low speed for three minutes. Because there is so little butter, you'll end up with a very fine crumb texture.
In a small mixing bowl, whisk together eggs, sour cream, oil, and vanilla extract until smooth.
Add the egg mixture to the flour mixture and beat on medium speed until just combined.
Slowly add milk and mix on low speed until just combined. The batter will be liquid. (Don't worry, you didn't do anything wrong. It's supposed to be that way.)
Fill cupcake liners just over 1/2 full.
Bake for 10 minutes and then test to see if they are done. They are done when a toothpick comes out without wet batter stuck to it. The cupcakes should appear white with specks of vanilla bean. They should not turn a golden brown. If they are not done, test again in two minutes. If they are still not done, test again in another two minutes.
When the cupcakes are done, remove them immediately from the tins and leave them on a cooling rack (or just on your counter if you don't own a cooling rack) to cool.
(This recipe is a mini-sized version of my best vanilla cupcake recipe.)Use a vanilla bean for the best vanilla flavor. If you don't have one, you can leave off the bean and skip related instructions and the mini cupcakes will still be amazing!Use cake flour for a lighter crumb. If all you have on hand is all-purpose flour, that will work fine.Be careful not to over-mix the batter.Use room temperature ingredients - everything will mix together more smoothly that way.Because everyone's oven is just a bit different and your liners may be a slightly different size than mine, I highly recommend that you bake a test cupcake before baking your whole batch of mini vanilla cupcakes.