Hi Gaura, substituting butter for the oil can make for a slightly drier cake, since oil is 100% fat and butter only 80% fat. Of course, you will get more flavor with butter than you will with oil, and using whole milk in place of the water might also help compensate for the butter's lower fat cotent. Cocoa does absorb a lot of liquid, which is why chocolate cakes often call for the oil and water combo, which helps contribute to a moist chocolate cake. 

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