Preheat the oven to 180°C (350°F) and line 2 cupcake trays with 24 cupcake liners.
Add the oil, eggs, and sugar to a bowl of a stand mixer fitted with a whisk attachment. Beat on medium-high speed until combined, about 2 minutes.
Sift together the salt and flour and add the flour mixture to the egg mixture. Beat at low speed until just combined.
In a jug combine the milk and vanilla, and slowly pour this into the cupcake batter with the mixer's motor running at low speed. Scrape down the sides of the bowl and mix again, making sure it is lump-free.
Add the passionfruit. Scrape the seeds and flesh from 3 passionfruit and mix this into the cupcake batter.
Fill 24 cupcake liners with the cupcake batter, filling them ¾ of the way up the liner. Bake in a preheated oven at 180C (350F) for 12 – 15 minutes or until a toothpick inserted into the center comes out clean. Cool completely on a wire rack.
Passionfruit Center
In a small saucepan set over medium heat, add the castor sugar, water, and passionfruit pulp. Heat until the sugar has melted, remove from the stove and allow to cool.
Passionfruit Buttercream
In a bowl of an electric mixer fitted with a paddle attachment, add the butter and icing sugar. Beat at medium speed until the mixture looks like fine breadcrumbs.
In a jug combine the milk and vanilla, with the mixer's motor running at low speed slowly add the milk mixture. Continue until it has all been added. Then increase the speed of the mixer to medium-high. Beat until light and fluffy, scraping down the sides of the bowl often.
Add the flesh of 3 passionfruit to the whipped buttercream, and beat again until combined. Place the buttercream into a prepared piping bag fitted with a round 1cm (0,5inch) diameter nozzle.
Pipe a swirl of frosting onto each cupcake, creating a recess in the middle for the passionfruit puree. Use a spoon to drip about 2 teaspoons of passionfruit puree into the center of each cupcake. Enjoy!