Mix a cup of almonds and a cup walnuts and place them on the baking sheet. Roast them in the oven at 180ºC (350ºF) for 10 minutes. Let them cool down a bit and then finely grind them in a food processor.
Beat the eggs with the sweetener and add sea salt. Melt the butter with coconut oil and slowly add to the eggs while beating constantly.
Add citric acid. (If you decide to use lemon juice, add it at this point). Now add vanilla extract and natural rum aroma. (If you cannot find rum aroma, you can use regular dark rum, the alcohol will evaporate during the baking) Add a tablespoon of psyllium powder and baking soda. Mix well.
Start adding ground nuts spoon by spoon. Slowly add sparkling mineral water. Now, add 1 tbsp of coconut flour and mix well. If the batter gets too thick, add just a bit more sparkling mineral water. In the end, add finely grated lemon zest and mix well.
Grease a 9" (30 cm) round cake pan with some coconut oil and pour in the batter. Preheat the oven to 170ºC (330ºF) and bake the cake for 60 minutes. Let it cool down a bit before taking it out of the oven. Then leave it in a cool place to cool down completely.
Warp a coin in aluminium foil and gently push it into the cake.