Preheat oven to 170°C and line a 20cm square cake tin with baking paper.
Place egg whites in a mixing bowl and beat with an electric mixer until stiff peaks form. Set aside.
Place egg yolks, sugar and lemon rind in a large bowl and beat with an electric mixer until light and frothy.
Add ¼ cup plain flour and beat with electric mixer on low until just combined. Keep the mixer running and slowly add melted margarine and lemon juice, followed by another ¼ cup of plain flour.
Heat the coconut milk in the microwave for 30 seconds or until lukewarm. Add to the mixing bowl with remaining flour and beat on low speed until combined.
Add the egg whites to the bowl and use a spatula to gently fold through the mixture. Pour into lined cake tin and bake in preheated oven for 35 minutes or until a golden crust has formed on top of your cake.