Preheat oven to 360 degrees F/ 180 degrees C. Line two 7 inch/ 18 cm cake pans with circles of parchment paper. Grease well the pans inside.
In a food processor grind the walnuts finely together with the xylitol.
In a large bowl beat the egg whites with an electric mixer at a high speed until thick and foamy.
In another mixing bowl beat the egg yolks at high speed until pale. Start adding from the walnuts to the yolks, beating with the mixer at a lower speed. When the mixture becomes thick, switch off the mixer. Start adding also from the egg whites and mix gently with a rubber spatula. Alternate with the walnuts, cocoa powder and coconut flour, folding carefully to maintain as much air as possible inside the batter.
When all ingredients are absorbed and the batter is smooth, divide it in three or four equal parts. Bake two parts together in the prepared pans for about 20 minutes and keep the rest of the batter in the fridge. Test the layers with a toothpick: when it comes out clean, take the pans out of the oven. Let cool for 15 minutes and then remove the layers from the pans. Line the pans with new circles of parchment paper and bake the rest of the batter.
Let all the layers (3 or 4) cool until room temperature before assembling the cake.
Break the chocolate into small pieces and place it in a heatproof bowl.
In a small saucepan heat the cream together with the xylitol until it starts simmering. Pour it on top of the chocolate. Let sit for 5 minutes and then start whisking, until the mixture becomes smooth.
Let the ganache cool until room temperature, whisking from time to time. Cover with cling film and refrigerate for 1-2 hours.
Beat the cold ganache with an electric mixer at a high speed until fluffy.
To assemble the cake:
Place one walnut layer on a serving plate and cover it generously with ganache. Place another layer on top. Continue the process until all layers are used. Leave about 1/3 of the ganache to frost the cake all around.
Decorate with chopped walnuts and chocolate. Alternatively leave aside some ganache to pipe on top.
Instead of xylitol you can use erythritol or stevia
When choosing chocolate for this recipe, check the carb content on the package. Use only chocolate with not more than 10-15% of carbohydrates as otherwise the dessert might not be keto friendly.
You can make a non keto version of the cake by using sugar instead of low carb sweetener. In this case the carb content of the chocolate is not important.