First, make the topping. Strain the yogurt using a cheesecloth or milk bag for at least 2 hrs or overnight. Whisk the strained yogurt with the vanilla extract and xylitol. Keep chilled while you make the sponge.

Heat the oven to 160C/180C fan/gas mark 4. Soak a sheet of baking parchment with water, then crumple it and wring it out. Use it to line a 22cm springform tin. In a clean, dry bowl, whip the egg whites to stiff peaks.

In a medium bowl, combine the yogurt, egg yolks, coconut oil and vanilla extract. Add the almond flour, xylitol, sunflower seed flour, baking soda and apple cider vinegar. Gently fold the whites and baking powder into the mixture.

Pour the batter into the prepared tin. Bake at 180C/160C fan/gas mark 4 for 15 mins, then reduce the temperature to 160C/140C fan/gas mark 3 and bake for a further 15 mins. The cake is ready when a toothpick inserted in the middle comes out a bit sticky but without batter clinging to it. Keep an eye on the cake during the last few minutes because it can brown suddenly.

Remove from the oven and leave to cool completely before releasing it from the tin and placing it on a serving platter. Spread the icing over the top and serve. Will keep chilled for up to three days.