If you are seeking moisture my suggestion would be to somehow add sugar; either directly or indirectly through the addition of a packet of instant pudding. Back in the early days on this board someone had responded to something I had posted and noted that sugar is hygroscopic
, and thus if you increase the amount of it in a cake you should increase the cake's moisture.
Granted other factors play a role---current climate (you aren't in a high altitude are you?) and weather conditions, age of the ingredients, baking time, oven temperature (use an oven thermometer!)---but those factors held constant I believe you would note a difference if you added some sugar. Also, some of the best chocolate cake recipes have sour cream in the batter so since you are working with white chocolate (though technically
not true chocolate) you might see a similar benefit.