In a deep mixing bowl, beat eggs with melted butter, salt, stevia, vanilla extract and lemon juice for 5 minutes. Add sour cream and keep beating.
Mix all the dry ingredients and add them spoon by spoon to the wet ingredients to create a thick batter.
Line an 8x11" (20x30 cm) baking pan with parchment paper, grease it with some butter or coconut oil and pour in the batter. Bake at 170ºC for 25 minutes. Let the cake cool down and cut it in half.
For the filling, beat the mascarpone with sparkling mineral water until you get a whipping cream consistency. Add stevia, sea salt, organic cacao powder and finely ground walnuts. Natural walnut aroma is optional, but you can also use 1 tsp of walnut liqueur.
Place one part of cake on a long plate, cover it with the filling, then place the other part of cake on top. Cover the whole cake with the rest of your filling/icing.
For the decoration, mix mascarpone with vanilla extract and use Decorate the Icing tool of your choice. Top with homemade chocolate fat bombs which you can also grate on top to create chocolate shavings, like in the photo.