Preheat oven to 180C/350F/gas 4. Weigh the eggs in their shells and measure out the same amount of butter and flour. Beat the butter and xylitol together for a few mins until pale and fluffy (an electric hand whisk or stand mixer will be easiest). Crack the eggs into a measuring jug and beat lightly. Slowly add the egg mixture to the butter and xylitol, a little at a time, whisking constantly so the egg gets completely mixed in before adding more. If it starts to "curdle" (get bitty or lumpy), add a tablespoon of the already weighed-out flour and continue adding more egg.

Once the egg is incorporated, stir in the vanilla extract and dump all the flour in in one go. Use a spatula to mix the flour in until there are no more visible specks of flour. Pour into a greased and lined 6" cake tin or two 7" sandwich tins and bake in the preheated oven for 25-35 mins for the 6" tin or 20-25 mins for the sandwich tins, or until the cake is golden brown and a skewer inserted into the centre comes out clean. Allow to cool completely.

If using one tin, use a bread knife to carefully cut the cake horizontally through the centre to make 2 equal sized cakes. Whisk the cream to stiff peaks, taste and sweeten with vanilla extract or liquid stevia, if you like. Spread on the bottom cake, then drizzle over 2-3 of the passionfruit fillings. Top with the other cake and dust with icing sugar (or sugar free icing sugar), if you like. Pour over the filling of the last passionfruit. Serve cool on its own or with extra fresh fruit like sliced bananas or strawberries (keep in the fridge for 1-2 days if it's not all gone that day).