Preheat the oven to 350F degrees.
Line 2 muffin tins with muffin papers (you'll end up with about 16-18 cupcakes total).
In a large bowl beat together the oil, sugar, eggs, and vanilla. Or whisk by hand
Beat (or whisk) in the peanut butter, ensuring that there are no lumps of brown sugar.
In a separate bowl, whisk together the flour, baking soda and salt.
In a liquid measuring cup, gently whisk together the sour cream and milk with a fork.
Carefully whisk about 1/2 of the flour mixture into the peanut butter mixture, followed by about 1/2 of the sour cream and milk mixture. Repeat the process until everything is mixed together. Its important to do this step by hand to avoid over mixing the batter.
Spoon into the prepared cupcake liners filling each about 2/3 to 3/4 full. You should end up with about 16 cupcakes total. If you only make 12 cupakes, then you've filled the liners too full.
Bake for 20-24 minutes, or until an inserted toothpick comes out clean.