Preheat oven to 160 °C (320°F) with the fan on (see note 4 if you don't have a fan function) and grease and/or line two 8 inch cake tins (I use my homemade cake release). In a bowl, sift together your flour, cornflour, baking powder and salt. Using a whisk or fork, mix until well combined. Set aside.
In a small bowl combine 3/4s of a cup (195g) of the strawberry filling, milk and gel food colour. Mix until well combined. Set aside for now.
In another bowl, add in your butter, vegetable oil and sugar. Using an electric mixer (hand or stand mixer are both fine - see note 2), cream together for 3 minutes until light and creamy.
Add in egg whites in 3 batches on a low-medium speed, mixing well in between each addition (about 10-15 seconds between each batch).
Add in half of your strawberry/milk mixture from earlier, strawberry essence/extract, vanilla and white vinegar. Mix until well combined. Now set your mixer aside as the remainder of the batter will be finished by hand.
Finish off by adding in half of your premixed dry ingredients to your wet mixture, and gently fold it in with a spatula until just combined. Then add in the remaining strawberry/milk mixture, and fold it in until just combined. Lastly, add in the remaining dry ingredients and gently fold it into the mixture until just combined. Do not overmix (see note 3). If your batter seems a little dull in colour, then you can add another drop of red gel food colouring.
Distribute the batter evenly into the two 8 inch cake tins, and bake for 35 minutes or until a toothpick comes out clean.
Once baked, allow the cake layers to cool in the cake tins for about 15-20 minutes, and then turn them out onto a wire rack to completely cool before frosting with the strawberry cream cheese buttercream recipe below.