Grease and line a 23 cm (9 inch) round deep cake tin.
Beat together the butter and sugar in a large bowl until light and fluffy, then beat in the egg yolks, buttermilk, polenta, flour, baking powder, coconut milk powder and desiccated coconut.
Whisk the egg whites in a large clean bowl until they form soft peaks, then fold into the cake mixture with the puréed mango.
Spoon the mixture into the prepared tin and place in a preheated oven, 200°C (400°F), Gas Mark 6, for 45–50 minutes until golden and firm to the touch. Remove from the oven and transfer to a wire rack to cool.
Slice the cake in half horizontally, when cool. Beat together the filling ingredients in a bowl and use half to sandwich the cakes together. Smooth the remaining mixture over the top.