Preheat oven to 180°C. Grease a 6cm-deep, 25cm heart-shaped cake pan. Line base with baking paper.
Using an electric mixer, beat butter and caster sugar until light and fluffy. Add eggs. Beat until combined. Add coconut, flour and buttermilk in 2 batches, stirring after each addition. Pour mixture into prepared pan. Bake for 40 to 50 minutes or until a skewer inserted into the centre comes out clean. Cool in pan for 5 minutes. Turn onto a wire rack to cool completely.
Make passionfruit icing: Sift icing sugar into a bowl. Add butter and passionfruit pulp. Stir until icing is smooth and spreadable.
Using an electric mixer, beat cream and icing sugar until soft peaks form. Stir in mango. Cut cake in half crossways. Spread cream mixture over base. Sandwich with cake top. Spread passionfruit icing over cake top. Serve.