Start by making the raspberry filling, as it needs to be completely cool before adding it to the cake. In a small saucepan, combine raspberries, sugar, and lemon juice over medium heat. Cover and let cook for about 3 minutes until the berries soften and release their juices. Stir occasionally as the mixture begins to simmer, breaking down some raspberries as you go.

In a separate small bowl, mix water with cornstarch, then add this to the simmering raspberry mixture. Stir continuously until the sauce thickens and bubbles gently. Remove from heat, and if desired, strain through a fine mesh sieve to remove seeds—though keeping the seeds can add a nice texture. Allow the filling to cool completely and chill in the refrigerator for at least 30 minutes before use. It can be stored covered in the fridge for up to a week.

Preheat your oven to 350°F (175°C). Line the bottom of two 8-inch round cake pans with parchment paper. There’s no need to grease the sides if you prefer the batter to cling to the pans, aiding in achieving taller layers; however, you may lightly grease if preferred.

In a medium bowl, sift together the flour, baking powder, baking soda, and salt. Whisk to combine evenly. In a separate large bowl, beat the softened butter, sunflower oil, sugar, and vanilla extract using an electric mixer for about 2 minutes, until the mixture is pale and fluffy. Beat in the whole egg until fully incorporated, then add the egg whites and beat until smooth. Mix in the sour cream.

Gradually sift in half of the flour mixture, stirring gently, then add half of the milk. Repeat with the remaining flour mixture and milk, beating on high speed for about 5 seconds at the end to ensure a smooth batter. Gently fold in the fresh raspberries.

Divide the batter evenly between the two prepared cake pans. Bake for 25-30 minutes, or until the tops are lightly golden and a skewer inserted into the center comes out clean. Allow the cakes to cool in the pans on a wire rack for 20-30 minutes. Carefully run a knife around the edges to loosen the cakes, invert them onto the racks, and remove the parchment paper. Let the cakes cool completely.

Place chopped white chocolate in a large bowl. Heat ½ cup of cream in a saucepan over medium heat with scraped vanilla bean seeds (add the pod too, as it contains additional seeds). Bring the cream just to a simmer, then immediately pour it over the chocolate. Cover the bowl and let sit for 2 minutes. Uncover and whisk until smooth. Let the mixture cool for 5 minutes, then whisk in the remaining cold cream. Cover and chill the frosting base for at least 2 hours or overnight until fully cold. If chilling overnight, cover after 20 minutes.

Once thoroughly chilled, add vanilla extract and beat the mixture with an electric mixer on medium speed until thick and fluffy, which should take around 2 minutes, depending on your mixer.

Place one cake layer on a serving plate. Transfer about 1 cup of frosting to a piping bag and pipe a border around the edge of the cake to hold in the raspberry filling. Spread 1 cup of raspberry filling within the frosting border using an offset spatula. Align the second cake layer on top. Cover the entire cake with the remaining frosting, smoothing the sides and top with an offset spatula. Decorate with fresh raspberries and, if desired, swirl in small dollops of the remaining raspberry filling for a beautiful finish.