Preheat the oven 170C / 338F degrees and line a 20 centimetre springform cake pan with baking paper.
To make the sponge, separate the eggs into two large bowls. Whisk together the egg yolks, vanilla extract, xylitol and salt until thoroughly combined.
In a separate bowl, sift together the coconut flour, almond flour, cocoa and baking powder. Ensure you break down any lumps in the flours.
Add the cream of tartar to the egg whites and using beaters or a stand mix, whisk the egg whites to stiff peaks.
Combine the sifted dry ingredients to the egg yolks and whisk until smooth. Add the melted butter and mix to incorporate evenly.
Fold in the beaten egg whites, a quarter of the whites at a time. Fold as gently as you can to preserve the air and create a fluffy sponge.
Gently spoon the mixture in to the prepared cake tin and bake for 35 - 40 minutes until a skewer inserted into the centre comes out clean. Remove and allow to cool on a wire wrack.
Once cool, cut the cake in half. Whip together the cream, xylitol and vanilla until firm and spread half in between the two pieces of cake and spread the remaining half on top. Decorate with fresh strawberries.