Preheat the oven to 180°C/350°F/Fan 160°C/Gas Mark 4 and grease line a springform tin with baking paper.
Whisk the sugar and margarine until thick and creamy. One at a time, add the eggs, whisking in between each one. Add some flour if it begins to curdle.
Fold in the flour and then mix in the desiccated coconut and milk.
Pour into the baking tin and spread out on top. Bake in the oven for 50 -55 mins. Then leave to cool in the tin for 5 minutes before removing then leave to cool completely on a wire rack.
If you want to use the air fryer, set at 160°c and bake for 50 mins or set the temperature higher for a quicker cooking time.
Meanwhile, make the buttercream.
In a bowl, mix the butter until loose and smooth and add the sieved icing sugar. Mix together, then add the coconut milk. Mix until you have a smooth buttercream. Add more milk if it's a bit thick.
Cut the cake in half and spread the buttercream mixture evenly on one sponge. Sandwich together and serve.
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