Beat the eggs with sea salt, lemon zest and sweetener. Slowly start adding melted butter and keep beating for at least 5-8 minutes. It will thicken.
Add coconut flour spoon by spoon and keep mixing. Now add psyllium powder. The batter will thicken almost to a dough consistency. Add sparkling mineral water and baking soda.
In the end, add lemon juice and keep mixing for 5 more minutes. You can add natural rum flavouring or any flavouring of your choice, for example, vanilla or cinnamon.
Grease a 25x35 cm (10x13 inch) pan with some coconut oil and then sprinkle with coconut flour to prevent sticking. Pour in the batter and bake in the oven for 25 minutes at 170ºC (340ºF). When it's bake let it cool down totally.