1. Heat your oven to 180°C. Use a little of the melted butter to grease two 20cm sandwich tins. Dust them lightly inside with flour.
2. Put the eggs and sugar into a heatproof bowl and place over a pan of simmering water. Whisk, using an electric hand-held whisk, for at least 7 minutes until the mixture is trebled in bulk, very pale and thick enough to hold a trail when the beaters are lifted. Sift the flour and salt together over the mixture and fold in gently, keeping in as much air as you can. Finally, very carefully fold in the remaining melted butter.
3. Divide the mixture between the prepared sandwich tins and bake for 25 minutes until golden and slightly shrunk from the sides of the tin. Leave the sponges to cool slightly in the tins for about 5 minutes, then transfer to a wire rack to cool.
4.Meanwhile, put 200g of the strawberries into a blender with 2 tbsp of icing sugar and blitz to a smooth purée. It should be intense and sweet; add a little more icing sugar if necessary.
5. Whip the cream until it holds soft peaks. Roughly fold in the strawberry purée, keeping the mixture rippled rather than a uniform pink (or you can simply drizzle the strawberry purée onto the cream as you assemble the cake).
6. Slice each sponge in half horizontally. Lay one of the sponges, cut side up, on a serving plate. Spread a third of the strawberry cream on top, then put a quarter of the blueberries and raspberries around the edge. Add a second layer of sponge and filling, then repeat to create the third layer. Place the final layer of sponge on the top and dust generously with icing sugar.
7. Arrange the remaining strawberries, blueberries and raspberries on top of the cake. Dust lightly with icing sugar and serve.