Many cake recipes (like this one) included instructions to alternate adding the wet and dry ingredients to the creamed butter and sugar. Believe it or not, this is for a good reason.

Adding the liquid all at once could saturate the creamed butter mixture and cause the batter to separate. Adding all of the dry ingredients will cause the batter to be thick and likely result in over mixing, which will yield a tougher cake. Instead, add them in two or three additions, beginning and ending with the dry ingredients. Stir ingredients after each addition, but only until just combined.