In the bowl of a heavy-duty electric mixer fitted with the paddle attachment, beat the butter and sugar on medium speed until light and fluffy, about 3 to 5 minutes. With mixer running, add eggs and vanilla extract; beat until combined.
With mixer on low speed, add 1 cup reserved flour mixture; mix until combined. Add 1/2 cup buttermilk; combine. Alternate adding flour mixture and buttermilk, ending with flour, until all has been incorporated.
Divide batter evenly between baking cups. Bake until light golden brown and a cake tester inserted into middle comes out clean, 25 to 30 minutes. Rotate pans halfway through baking. Transfer pans to a wire rack to cool.