In a medium-sized mixing bowl or bowl of a stand mixer, mix together brown sugar, cake flour, baking powder, baking soda, and salt.
Add butter and mix on medium-low speed for three minutes.
Mix in eggs, one at a time until just combined.
Mix in sour cream and vanilla until just combined.
Slowly add milk and mix on low speed until just combined. The batter will be liquidy. (Don't worry, you didn't do anything wrong. It's supposed to be that way.)
Fill cupcake liners 1/2 full. DO NOT OVERFILL. These cupcakes are meant to be flat. If you fill the liners too high, the tops will spread like pancakes instead of doming.
Bake for 14 minutes and then test to see if they are done. They are done when a toothpick comes out without wet batter stuck to it. 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.
Frosting Instructions
Beat butter on high speed with an electric mixer for about three minutes until light and airy.
Mix in powdered sugar a little bit at a time.
Add salt and butterscotch sauce and continue to beat for another minute.
Pipe or spread onto cooled cupcakes and drizzle with extra sauce as desired.
Video
Notes
Be careful not to over-mix the batter.
Use room temperature ingredients - everything will mix together more smoothly that way.