Preheat the oven to 160°C and line a 23 cm x 23 cm cake tin with parchment paper.
Separate eggs whites from yolks.
Whisk egg whites and 1 pinch of salt with electric whisk or hand in a very clean bowl, until is firm, consistency similar to whipped cream and are thick and glossy.
Whisk egg yolks, honey and 1 pinch of salt with electric whisk until is foamy, its volume is doubled, and its color is light yellow.
Add whisked egg whites and baking soda to egg yolks mixture and mix very carefully. Do not break the egg whites!
Add coconut flour to the mixture spoon by spoon and mix it very gently.
Pour lemon juice into the batter and mix it carefully.
Pour the batter into the lined cake tin and bake for 30 minutes. Stick a toothpick into the middle of the cake and if it comes out clean, the cake is done, if it is not clean, bake it 5 minutes more and try again.
Wait until the lemon sponge cake is completely cool then cut half.
Grease one half with lemon curd then place the other half on it.
Place the chocolate with coconut oil in a heatproof bowl over a pan of simmering water. The base of the bowl should sit above the water without touching it (Bain Marie). Allow the chocolate to melt.
Pour the melted chocolate over the filled lemon sponge cake and decorate with lemon and lime zest.
Put the cake into the fridge for 1 hour until the chocolate is hard on the top.
Slice it with warm knife and the chocolate will not break.