Sift flour, baking soda, cinnamon and salt in a bowl.
Mix together in a separate bowl, the oil, buttermilk, sugar, vanilla and eggs. Add flour to wet mixture. Mix in pineapple, carrots, coconut and walnuts.
Line 2 9-inch round cake pans with butter and dust with flour. Pour in batter and bake for 350 degrees for 55 minutes.
Chop white chocolate into fine pieces and place in a metal bowl that has been placed over simmering water. Stir until chocolate begins to melt. Stir until smooth. Allow to cool to room temp, stirring occasionally to keep smooth.
In a mixing bowl, beat the cream cheese with flat beating attachment until smooth and creamy. Gradually beat in cooled white chocolate until smoothly incorporated. Beat in butter and lemon juice. Use at once to ensure smoothness.