Preheat the oven to 350ºF. Grease and line three 4-inch or one 6-inch cake pan.
In the bowl of your stand mixer (a handheld mixer works fine too!), cream together the butter and sugar on medium-high until light and fluffy, 3 min. Turn the mixer to low and add the egg, mixing until just combined. Add the vanilla and sour cream, turn the mixer to high, and beat for 1 min.
Turn the mixer to low speed and add the dry ingredients all at once. Mix until just combined, then slowly pour in the milk. Continue to mix on low speed until just combined, about 30 seconds.
Pour the batter evenly between the prepared cake 4-inch pans (or all into one 6-inch cake pan). If using 4-inch cake pans, bake for 18-22 minutes. If using a 6-inch cake pan bake for 24-28 minutes. The cake is done when it springs back to the touch and a toothpick inserted comes out with a few moist crumbs.
