5. Make the Batter
In a stand mixer fitted with the paddle attachment, add the butter and light brown sugar. Mix on medium-high speed for 6 minutes or until light and fluffy, scraping down the sides of the bowl as needed to evenly mix the ingredients. Turn the mixer to medium and add the vanilla, egg, and buttermilk, and mix for 1 more minute or until combined. The batter will look curdled at this point. Add the dry ingredients to the batter in 3 parts, scraping down the sides of the bowl as needed. After adding all the dry ingredients, mix for about 1 more minute on medium speed until smooth. Add the mashed potato and mix again for 1 more minute, until smooth.