Preheat oven to 180 C / Gas 4. Grease a 20x30cm tin. Sieve together the flour, bicarbonate of soda and cinnamon. Set aside.
In a large bowl, combine the sugar and oil until blended. Beat in the eggs one at a time, then stir in the vanilla. Stir in the flour mixture just until incorporated. Fold in the pineapple, carrots, coconut and walnuts. Pour batter into prepared tin.
Bake in the preheated oven for 45 minutes or until a skewer inserted into the centre of the cake comes out clean. While still warm, poke holes in the cake with a fork.
In a saucepan, combine butter, sugar, buttermilk and vanilla. Bring mixture to the boil over medium heat, then reduce heat and cook for 5 minutes. Pour glaze over warm cake, then allow to cool completely.