Preheat the oven to 180 C / Gas mark 4. Grease and flour two 23cm (9 in) round cake tins or one 23x33cm (9x13 in) tin.
In a medium bowl, cream together caster sugar and 125g butter. Mix in eggs and 3 mashed bananas.
Sift together flour and bicarbonate of soda in a separate bowl. Add to the creamed mixture alternately with buttermilk, mixing well after each addition. Blend in 1 teaspoon vanilla extract. Fold in 125g of pecans and 75g coconut. Pour batter into prepared tins.
Bake 45 to 50 minutes in the preheated oven. Cool completely before icing with Banana Nut Icing.
To make the icing: Cream together 125g butter and icing sugar until light and fluffy. Mix in 1 medium mashed banana, 125g pecans, 75g coconut and 1 teaspoon vanilla. Use to ice the cooled cake.