No!!! don;t use a box. If you have a mixer of any type,sponge is easy. even without, if you have a good arm. And i don;t think you can get a box sponge cake. A sponge doesn;t have any fats except the egg yolks, and it's sufficiently rubbery to be able to be rolled up without breaking. A butter cake ("yellow cake") would break.
I do the most simple method, without separating the eggs, and i get it into the oven, start to finish, in less than ten minutes.
the basic proportions are 6 eggs, 1 cup of sugar and 1 cup of flour. If you want to make a 15 X 10 it should be 5 eggs, and i can;t stand to do math and don;t have the time, but maybe you can, just make it proportionately less. This cake is good for a very large roasting pan, 18x13x1
Beat the whole eggs, till they;re very fluffy and light. Gradually add sugar while beating - just a little at a time, and keep beating till when you lift the beater from the mixture it forms a slowly-dissolving ribbon, as julia child used to say.
Sift the flour over it (I rarely sift and it comes out fine - just stir the flour with a whisk if it has lumps) and fold it in gently, go down in hte center with the spatula, drag it along the bottom towards you and twisting the spatula to lift the egg mixyture, dump the egg stuff on top of the flour, then turn 1/8 and do the same, and continue all around the bowl. YOu want to make sure the flour is all mixed in, and you don't see any white as you turn over the batter.
that;s it. Easier than that they don;t come!
To bake, heat oven to 350, grease a large flat pan (1 inch high) - I have one 18 x 13 x 1 and this cake fits it perfectly - then lay a sheet of parchment paper in it, and grease again. Pour in the batter. bake, checking every so often - after about 20 min it might be done. If it looks liquidy in the middle, leave it a little more, then test by touching in the center. It should spring back.
Remove from oven, dust generously with powdered sugar using a sifter or seive and then lay a cloth on top. Put a flat pan on top, and turn all over together. Remove pan, remove paper, being careful not to break the edges of thye cake, then roll up with the cloth inside. Let it cool completely before filling. If you want to freeze, you will have to fill it first. Then wrap well and freeze.
VariationsL grind nuts (walnuts, almonds, filberts) and substiutute some or all of the flour with the ground nuts. (I had a great hungarian cake made this way, but with layers, though i do it often as a roll. (though this is not a buche de noel - more summery a taste than that) You fill it with whipped cream mixed with raspberry jam. Make the sponge with ground walnuts.
Substitute half the flour with cocoa powder.
I wonder if anyone knows what difference it makes if you beat the whites and yolks together as opposed to beating separately. I don;t notice a difference.