Preheat the oven 170C / 338F and line a 20 centimetre square 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, vanilla protein powder and baking powder.
Add the cream of tartar to the egg whites and using beaters or a stand mix, whisk the egg whites to stiff peaks.
Add 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 25 - 30 minutes until a skewer inserted into the centre comes out clean. Remove and allow to cool on a wire wrack.
Meanwhile, make the strawberry chia seed jam using the link above.
Once the cake is cooled, cut in half horizontally and then into rectangles. Spread the jam between each rectangle and sandwich together.