Hi, I follow the recipe for Lindy Smith found in her cake decorating bible, she uses a Madeira cake, if you go onto her website there are some tips for keeping it moist, adding a quarter of a teaspoon of glycerine per egg used, also I leave the cake wrapped for a day before cutting, I use syrup once I have cut the cake at the time I am buttercreaming, I also trim the outside of my cake once I have split it and butter creamed it. Hope this helps, her chocolate & also chocolate fudge cake have always gone down well, everyone’s says they are moist all these cakes are, if I use a Victoria sponge cake I tend to white chocolate ganache it rather than buttercream it.