Preheat oven to 350 degrees.

Grease two 9" round cake pans, then dust with flour; or line cupcake pans with papers.

Place the cake mix, milk, melted butter, eggs, and vanilla in a large mixing bowl.

Blend with an electric mixer on low speed for 1 minute. Scrape down the sides of the bowl with a rubber spatula.

Increase the mixer speed to medium and beat 2 minutes more, scraping the sides down again if needed.

The batter should look well blended.

Divide the batter between the prepared pans, smoothing it out with a rubber spatula.

Place the pans in the oven side by side.

Bake the cakes until they are golden brown and spring back when lightly pressed with your finger, 27-29 minutes (a few minutes less for cupcakes).

Remove the pans from the oven and place them on wire racks to cool for 10 minutes.

Run a knife around the edge of each layer and invert onto a rack, then invert again so the cakes are right side up.

Allow them to cool completely, 30 minutes or more, before frosting.