Preheat the oven to 170C/325F/Gas 3.
Take a very large mixing bowl, put the flour and baking powder in a sieve and sift it into the bowl, holding the sieve high to give it a good airing as it goes down. Now all you do is simply add all the other cake ingredients (except the icing sugar) to the bowl and, provided the butter is really soft, just go in with an electric hand whisk and whisk everything together until you have a smooth, well-combined mixture, which will take about one minute. If you do not have an electric hand whisk, you can use a wooden spoon, using a little bit more effort. What you will now end up with is a mixture that drops off a spoon when you give it a tap on the side of the bowl. If it seems a little too stiff, add a little water and mix again.
Now divide the mixture between the two tins, level it out and place the tins on the centre shelf of the oven. The cakes will take 30-35 minutes to cook, but do not open the oven door until 30 minutes have elapsed.
To test whether the cakes are cooked or not, touch the centre of each lightly with a finger: if it leaves no impression and the sponges spring back, they are ready.
Next, remove them from the oven, then wait about five minutes before turning them out on to a wire cooling rack. Carefully peel off the base papers, which is easier if you make a fold in the paper first, then pull it gently away without trying to lift it off. Now leave the sponges to get completely cold, then add the filling.
To make this, first slice the passion fruit into halves and, using a tsp, scoop all the flesh, juice and seeds into a bowl.
Next, in another bowl, combine the mascarpone, fromage frais, sugar and vanilla extract, using a balloon whisk, which is the quickest way to blend them all together. After that, fold in about two-thirds of the passion fruit.
Now place the first sponge cake on the plate or cake stand you are going to serve it on, then spread half the filling over it, drizzle the rest of the passion fruit over that, then spread the remaining filling over the passion fruit. Lastly, place the other cake on top, press it gently so that the filling oozes out at the edges, then dust the surface with a little sifted icing sugar.