If you wish to use standard 8-inch tins, my suggestion is to simply double the recipe. In an ideal world, you would scale up the recipe to bake it using 5 eggs, but that leads to some fiddly measurements. Making double is far easier - just ensure you do not fill your sandwich tins more than ⅔ full with the batter and use the leftover batter to make a few cupcakes (freezable). You perhaps will not need to double the buttercream though - I'd suggest making 1.5 times that stated in the recipe.