This recipe uses 3 x 6 inch circular cake tins. If you wish to use standard 8.5 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 2/3 full with the batter and use the leftover batter to make a few cupcakes. You perhaps will not need to double the buttercream though - I'd suggest making 1.5 times that stated in the recipe.