Heat oven to 190C/170C fan/gas 5. Brush 2 x 20cm cake tins with melted butter, line the bases with baking paper, then dust well with flour tipping out any excess. Set aside.
Put the sugar and eggs in a large heatproof bowl, then set it over a pan of barely simmering water. Whisk with an electric hand whisk for about 7 mins or until the mixture is pale and has trebled in volume. Remove from the heat, then 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, then pour into your prepared cake tins. Cook for 25 mins until the cake is golden and risen - a skewer pushed into the cake should come out clean. Allow the cakes to cool for a few mins in the tin, then remove and cool completely on a wire rack.
Meanwhile, whip the cream until it just holds its shape, then set aside. Whizz about a quarter of the strawberries with 1 tbsp sugar until smooth, then fold this puree through the cream so you get a ripple effect.
To assemble the cakes, cut each one in half horizontally. Put one sponge on a serving plate, spread a quarter of the cream mix on top, then dot a quarter of the blueberries and raspberries around the edge. Repeat this step two more times, put the last layer of cake on top (save a top half of cake for the top layer), spread the remaining cream over, then put the blueberries and raspberries around the edge and pile the strawberries in the centre. Dust with icing sugar just before you serve.