Soak the Hippophae berries in warm water and add 2 tbsp of heated rum. (you can heat it in a small saucepan, or just use a candle underneath your spoon) Cover it with a plate and keep it at room temperature.
Roast the almonds and walnuts in the oven for 10 minutes at 170ºC (140ºF). Let them cool down and then grind them finely using a food processor.
Beat the eggs using an electric mixer. Add sea salt, citric acid, stevia and baking soda.
Melt the butter and add it slowly to the egg mixture. Now start adding ground nuts spoon by spoon. Add citrus zest and vanilla extract.
At this point, add psyllium powder and coconut flour. The batter will get thicker now.
Start adding sparkling mineral water spoon by spoon. Beat the mixture for another 2-3 minutes. At this point add strained Hippophae berries to the mixture and mix with a spatula. Keep the water in which the berries were soaked.
Preheat the oven to 170ºC (140ºF). Grease a "Glamoutsa" pan (bundt pan) with coconut butter and sprinkle it with some coconut flour. Pour in the batter and place it in the oven.
Bake the cake for 50 minutes without opening the oven. When it's ready remove it from the pan and let it cool down totally. Now cut the cake in half horizontally.
Mix mascarpone with the remaining water in which you soaked the berries. Add stevia and vanilla extract.
Fill the cake with the Mascarpone mixture and cover it from all sides. Decorate the cake with some Hippophae berries, walnuts, almonds, coconut shavings and frozen berries of your choice.
Refrigerate for 2 hours before serving.