vkl, white cakes are naturally dry, because they are low in fat; they aren't supposed to have butter, oil or egg yolks in them which can change the color of them from pure white to off-white or very pale yellow.

If you truly want a very white cake, but moist, do what pro-pastry chefs do. Make the cake layers, baking them with a parchment liner in the pan. Trim and split them. Make a simple syrup (50/50 water and sugar, heated until sugar dissolves), and brush over the cake layers fairly heavily, so that when you press your finger down gently on the cake you can see the sugar syrup rise up to meet your finger. Set the cakes aside for a bit before icing.

Sugar syrup is not only a cure all for dry cakes, but also helps to preserve it a little bit longer. And yes, the icing will still stick to it. You can also flavor the syrup with a bit of kirsch, Gran Marnier, Annisette, Creme de Cocoa or any other clear or semi-clear liquor. Gives a bit more flavor to the cake.