Preheat oven to 175 ºC (350 ºF) and line a baking tray with greaseproof/baking paper. Use a shallow baking tray (I've used a 10 x 14" rectangular tray 1 inch deep).
Bake at 175 ºC (350 ºF) for about 20 minutes or until golden and a skewer/toothpick comes out clean.
Note 1: In making this cake, I've baked two sponges, each in a 10 x 14" rectangular tray 1 inch deep baking tray. I then trimmed the sponges to get rectangles, which I then halved to get 4 layers. You can just as well bake your sponges in round baking trays. Changing the amount or thickness of the sponge being baked will require you to adjust the baking time somewhat (use the toothpick test to determine when the sponge is baked). Note also that you can either make all the sponge mix at once and bake the two sponges one after the other, or make two separate sponge mixes (so that part of the mix doesn't stand around while the first part bakes).
Note 2: If you notice that the initial coatings of buttercream have a lot of sponge crumbs in them, crumb coat the cake before the final buttercream finish (that is, coat the cake in a this layer of buttercream and put it in the fridge for about an hour to seal the crumbs inside).