Preheat the oven to 170°C (340°F) and line 24 cupcake liners into two 12-hole cupcake pans.
Add the oil, eggs, and sugar to a bowl of a stand mixer fitted with a whisk attachment. Beat on medium-high speed until light and fluffy, about 4 minutes.
Sift together the salt and flour then add the flour mixture to the egg and sugar mixture. Beat at low speed until just combined.
Mix together the milk and vanilla extract in a jug, 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.
Pour the cupcake batter into the cupcake liners using a jug or ice cream scoop, filling each case two-thirds of the way up. Bake in a preheated oven for 15 to 20 minutes or until a cake tester inserted into the center of the cupcake comes out clean. Allow the cupcakes to cool in their tins for 5 minutes, then to cool completely on a cooling rack.
Raspberry Purée
In a small saucepan set over medium heat add the fresh raspberries, castor sugar, and lemon juice. Allow the fruit to soften and the sugar to melt. Stir occasionally with a spatula and press the fruit against the sides of the saucepan to help them break down if needed.
Once the sugar is melted and the purée has thickened, allow the mashed raspberry purée to cool to room temperature. Reserve ⅓ cup for the frosting and add the remaining puree to a piping bag for the cupcake filling.
Raspberry 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. Increase the speed of the mixer to medium-high. Beat until light and fluffy, scraping down the sides of the bowl often.
Add ⅓ cup of the cooled raspberry purée to the frosting and beat to combine. Add the frosting to a piping bag fitted with your chosen decorating nozzle.
Assembly
Use a cupcake or apple corer to remove the center of each cupcake, fill the center with the raspberry puree, and then pipe a swirl of frosting onto each cupcake. Decorate with the remaining raspberry purée and fresh raspberries.