Place a roasting tin in the bottom of the oven and fill with about 3cm (1in) water. Preheat the oven to 180℃ (160℃ fan)/350°F/gas mark 4. Grease and line a 18cm (6in) deep cake tin.
Beat the butter and sugar together until pale and fluffy. Add the eggs one at a time, beating well after each addition.
Sift in the flour and add the coconut. Carefully fold in. Spoon into the prepared tin and bake in the centre of the oven for about 1 hour to 1 hour 15 minutes until a skewer inserted into the centre comes out clean.
Allow to cool in the tin for about 15 minutes, then transfer to a wire rack to cool completely.
Make the buttercream
Beat the butter until pale, then add the icing sugar a third at a time and beat until combined. Add the coconut milk and beat until very fluffy.
Trim the top of the cake level if required then carefully cut into three equal layers.Spread about one third of the buttercream over one of the sponge layers. Place the next one on top and repeat with another third of the icing and the third layer.
Use the remaining buttercream to coat the top and sides of the cake. Pile raspberries on top and serve or use as the top tier of a wedding cake.