How to bake the Betty Crocker Super Moist White Cake
If you try to break this all up, it's possible that people will try to shut them down as being duplicates.  (and they close to a few others, but I don't think they're true duplicates)

I don't know if a 1.5&quot; high pan would be tall enough.  I've done what you're asking about, but in a 3&quot; high 8&quot; across pan, and it comes up to the top (at least, the dome does).  As an 8&quot; is about 80% of the area of a 9&quot;, so I'd expect it to rise to about 2.4&quot;.  See How much can I change the height of a cake? and  Correct way to join two halves of sponge cake?

Don't increase the number of egg whites.  You're already adding more water by changing the oil to butter.  (as butter is about 15% water).  Keep the egg whites the same.  See Conversion rule: how to switch oil and butter? and for the inverse, Can vegetable oil be used in place of butter? (which links to lots of others).

You can only get away with it if you're baking them all at the same time, as letting it sit before baking will change the texture.  Of course, that would require having multiple pans, and enough space in your oven to fit them all in without crowding.  (and related : Cakes cooked in same tin but come out different sizes )
But if you're going to be making 4 times the batter, I'd actually recommend making 3 cakes rather than cramming it all into two.  (although, I'd feel better if your pans were 2&quot; high.

Yes, lower the heat to reduce doming.  (also see How do you make a cake lift equally and minimize doming? ).  And you cook them until they're done.  (I will never bake a 14&quot; cake again if I can help it, as even with baking strips and a heat core, the outside was done so much sooner than the inside that I had to turn the oven down so far that it took hours to finish).


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Oh, and 'super moist' cakes aren't so great for stacking really high.  You typically want a denser cake (eg, pound cake), so the weight of the upper layers don't crush the bottom.  As they end to be dryer, you then pour on some flavored syrup to soak in.
There is a trick for dealing with boxed mixes though -- add a package (one of the smaller boxes) of instant pudding in with the cake mix, without adding any of the stuff that the instant pudding calls for adding.  But it'll change the color of your cake, if you were hoping for snow white.