Preheat the oven to 175°C (350°F) and grease, line, and lightly flour two 8” cake tins.
Sift the flour, baking powder, and sea salt into a large bowl, give it a quick whisk, and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar on a medium speed until fluffy and pale, then stir in the vanilla.
Add the egg whites, two at a time, mixing for 30 seconds and scraping down the sides of the bowl in between each addition.
Turn the mixer to a low speed and alternate adding the dry ingredients and milk: add one third of the flour mix, then one half of the milk, then the second third of the flour mix, then the second half of milk, and finally, end on the third portion of flour. Give the batter a scrape and one final 10 second whizz up.
Remove 250 grams of the batter into a separate bowl and fold in the large portion of raspberry puree.
Take one half of the vanilla cake batter and divide it between the two cake tins.
Take one quarter of the raspberry batter and make three dots of batter on top of the vanilla batter in the tin. Repeat this step with the second cake tin.
Divide the remaining vanilla batter between the two cake tins, taking care spread it evenly over the raspberry batter, not all in one dump or the swirl won’t work.
Divide the remaining raspberry batter between the two cake tins, dotting it in as in step 8.
Using a butter knife, swirl the batter in the cake tins in a vertical “M” formation. Rotate the cake tins 90° and repeat the swirling motion. Repeat this step twice more.
Tap the cake tins lightly on the counter to level the batter and then bake them for 24-27 minutes or until a toothpick comes out clean and the sponges spring back when touched.
Allow the cakes to cool in the tins for 10 minutes, then remove them to a wire rack to finish cooling completely.