Preheat the oven to 180 celsius (350 Fahrenheit) and grease and line a 22cm x 33cm (9 x 13 inch) swiss roll tin with baking paper.
Place the eggs, sugar and vanilla extract in a large bowl and whisk with an electric mixer until the mixture is extremely pale and creamy, around 5 minutes.
Switch the mixer off and remove the bowl and add the oil and milk and gently fold through with a metal spoon or spatula. Sift in the flour and salt and gently fold through, being careful not to knock too much air out of the whisked eggs. Finally, add the purple food colouring, if using, and fold through until evenly distributed.
Pour the mixture into the prepared tin and tilt to distribute evenly in the tin so the cake rises evenly. Bake for 10 to 15 minutes (check at 10 minutes), or until the cake has started to pull back a little from the sides, and springs back when touched.
While the cake is in the oven dust baking paper with icing sugar and when the cake is done remove from the oven and immediately turn out onto the icing sugar dusted baking paper. Carefully remove the baking paper from the back of the cake and immediately, beginning at the narrow end of the cake, roll into a tight roll. Place on a wire cooling rack, seam side down, to cool completely.
To make the curd, strain the seeds from the passionfruit pulp and discard the seeds. Place the passionfruit juice, egg yolks and sugar in a small saucepan and whisk until smooth. Place over a medium low heat and whisk until thickened. This will take between 5 to 10 minutes. The mixture will be ready when you can see a slight trail from the whisk in the mixture. It will thicken further when refrigerated. Refrigerate.
Fold the passionfruit curd through the cream until combined.
To fill, unroll the cake and spread the cream over the cake and re-roll. Place on a plate seam side down.
It's important to roll this cake while hot. If you leave the cake to cool first before rolling it's prone to crack when rolled.This cake is best eaten the day it's made.