However, it is still possible if you tweak the cake mixing method, which has to do with the sequence of adding ingredients. Canola oil can be used if you use the '1-stage' method instead of the creaming method. What's 1-stage?
Simply categorize ingredients into 2 main groups - powder and liquid, or sometimes also known as dry and wet. Powder or dry can be cake flour, sugar, cocoa powder etc. Liquid or wet can be egg, milk, puree, melted butter etc.
1-stage refers to the adding of ingredients in one stage, ie. first have all the powder or dry ingredients in a mixing bowl, then pre-mix all liquid or wet ingredients together and at one go add into the powder or dry ingredients. Then as in any mixing method for any type of sponge, once flour comes into contact with moisture, simply mix well to get all ingredients to combine well but never over mix.
So there you have it...instead of butter in a creaming-method recipe, substitute with canola oil and adopt a 1-stage cake mixing method.
Happy baking!!