Preheat your oven to 190C/170C fan/gas 5. Begin by brushing melted butter onto two 20cm cake tins, lining their bases with baking paper, and then dusting them thoroughly with flour, making sure to tip out any excess flour. Afterward, set the tins aside.
Take a large heatproof bowl and place the sugar and eggs inside. Next, set the bowl over a pan of barely simmering water. Use an electric hand whisk to beat the mixture for approximately 7 minutes, or until it has tripled in volume and become pale. Afterward, remove the bowl from the heat source and slowly pour in the butter, folding it in as you pour until it is completely mixed in.
Gently fold the flour and a pinch of salt into the egg mixture, and then pour the resulting batter into your prepared cake tins. Bake the cakes for 25 minutes or until they have turned golden and risen, and a skewer inserted into the cake comes out clean. Allow the cakes to cool for a few minutes in the tin, then remove them and let them cool completely on a wire rack.
Meanwhile, whip the cream until it forms soft peaks, and then set it aside. Take about a quarter of the strawberries and mix them with 1 tbsp sugar in a blender until the mixture is smooth. Next, fold this purée through the cream to create a ripple effect.
To assemble the cakes, slice each cake in half horizontally. Place one of the sponges on a serving plate, spread a quarter of the cream mixture on top, and then dot a quarter of the blueberries and raspberries around the edge. Repeat this step two more times, putting the last layer of cake on top (save a top half of cake for the final layer), spread the remaining cream on top, then arrange the blueberries and raspberries around the edge and pile the strawberries in the center. Finally, dust the top with icing sugar just before serving.