Heat the oven to 160C fan and line 3 x 6” loose bottomed cake tins with greased paper.
Place the strawberries in a blender and blend until smooth. Sieve any pips out and pour the liquid into a small saucepan. Add the lemon juice and heat on medium heat for 5-10 minutes until some of the water has evaporated, but the colour doesn’t go too dark. Pour into a jug and set aside to cool.
Once fully cooled, combine with the milk and mix well. Add a small drop or two of pink food colouring (to your desired brightness), mix again and keep nearby.
In the bowl of a stand mixer, mix the oil, butter, sugar and vanilla extract together until pale and fluffy.
In a separate bowl add the baking powder and salt to the flour and stir. Slowly add a couple of spoons of the flour whilst the mixer is on low speed and once most of the flour has mixed through add a ¼ of the strawberry milk liquid and combine. Alternate adding the milk and flour to the batter until fully incorporated. Switch off and keep nearby.
In another clean bowl, whisk the egg whites until stiff peaks form. Take a spoon of the egg whites and gently fold them into the cake batter. The first spoon will help the mixture loosen up before adding the remaining amount. Fold gently until you have an even batter.
Divide evenly between the tins, pop into the oven and bake for a minimum of 20 minutes. Check if cooked through otherwise continue to bake for longer until the sponge is springy or a toothpick comes out clean.
Remove from the oven, allow to cool before turning out onto a wire rack. Once completely cooled, place in the fridge to firm up and make icing easier.
Using a stand mixer, beat the butter on high speed until super pale and almost white. This should take around 5-8 minutes.
Slowly add the icing sugar and combine on medium/high speed until you have a smooth buttercream. Add a splash of vanilla extract and give it a final mix.
Keep aside around 3 tablespoons of the ‘white’ buttercream in a piping bag for later on. At the same time remove another 3 tablespoons of the buttercream into a small bowl and add a little bit of the strawberry puree so you end up with a pale pink colour (you may also want to add a very small amount of pink food colouring).
Take the refrigerated sponges out and trim the tops with a serrated knife to get a flat surface. Place a small dot of the icing onto a cake board and place a layer of sponge on top. Add an even layer of buttercream, and top with a layer of sponge. Do the same for the remaining sponge making sure you keep enough buttercream for the tops and sides.
Once all 3 layers have been stacked, apply a thin crumb coat of icing to the sides and top and smooth out using an offset spatula or a cake scraper.
Pop the cake back into the fridge for 15-20 minutes to let it set slightly, and then repeat making sure you have neat and even coverage around the cake.
Place this back in the fridge for another 10 minutes after which you can create a subtle watercolour effect using the pale pink colour you reserved earlier on.
Using a small spatula, smear small blobs of colour randomly around the cake. Then take a tall straight edged scraper and gently blend the pale pink into the ‘white’ buttercream. Apply extra smears of pink depending on how subtle you want the final design to be.
Take the piping bag and pipe 8 stars around the cake and top with halved strawberries.