Preheat your oven to 350. Butter/grease an 8x8 pan.
With a handheld mixer or in the bowl of a stand mixer, cream together the softened butter and sugar. Mix for at least 3 minutes, until light and fluffy.
Add egg and mix to combine. Don't forget to scrape down the sides of the bowl!
Add your vanilla, milk, and sour cream. Mix to combine, scraping down the sides of the bowl before adding dry ingredients.
Whisk together your dry ingredients in a medium bowl. Then mix into your butter/sugar mixture, stopping when there's a few streaks of flour left. Finish mixing by hand. This ensures you don't overmix!
Pour into prepared pan, spreading the batter as evenly as possible.
Bake for 25-30 minutes, or until a toothpick comes out clean. This is *exactly* 28 minutes in my oven!