Preheat the oven to 180°C (350°F) and line 2x 20cm (8 inches) round cake tins with oil and parchment paper.
Add the oil, butter, and sugar to a bowl of a stand mixer fitted with a paddle attachment. Beat on medium-high speed until light and fluffy, about 4 minutes.
Add the eggs to the butter and sugar mixture one at a time. Beat well between each addition and scrape down the sides of the bowl often.
Sift together the salt and self-raising flour and then add the flour mixture to the egg and sugar mixture. Beat at low speed until just combined.
Combine the milk and vanilla extract in a jug, and slowly pour this into the cake mixture 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.
Divide the cake batter between the two prepared cake pans and bake in a preheated oven for 40 minutes or until a cake tester inserted into the center of the cake comes out clean. Allow the cakes to cool in their tins for 30 minutes, before turning them onto a wire rack to cool completely.
Strawberry Filling
Add the chopped strawberries, lemon juice, and castor sugar to a medium saucepan set over medium heat.
Heat until the sugar has melted and the strawberries soften.
Mix together the water and cornflour in a small bowl and add this to the saucepan.
Allow the water from the fruit to evaporate and for the filling to thicken. Once the filling coats the back of a spoon, remove the pan from the heat and transfer the filling into a glass jar to cool completely.
Vanilla Buttercream
Add the butter to a bowl of an electric mixer fitted with a whisk attachment. Beat at medium-high speed until the butter is light in color and well whipped.
Sift the icing sugar over the whipped butter and beat again, at medium-low speed to begin with, and then increase the speed to medium-high. Beat until fluffy, about 4 minutes.
Add the cream and vanilla to the frosting mixture, and beat until light and fluffy, scraping down the sides of the bowl often.
Transfer the frosting to a piping bag and use immediately.
Decorating
Use a sharp knife to trim the domed tops of the cake layers if necessary.
Secure the first cake layer onto a cake plate with a bit of frosting. Pipe a layer of frosting over the surface of the cake and smooth this out with a spatula. Pipe a border of frosting around the circumference of the cake.
Spoon ¾ cup of the cooled strawberry filling into this border of frosting and smooth it out with the back of a spoon. Reserve the remaining strawberry sauce for serving.
Place the second layer of cake onto the first layer with the cut side facing down.
Pipe the frosting over the tops and sides of the cake and use a spatula or cake scraper to even this out. Place fresh strawberries on top of the cake, serve, and enjoy!