Preheat the oven to 180°C/gas mark 4 and line, grease and dust 2 x round 22cm cake tins with flour.
Put the sugar and eggs in a heatproof bowl and sit over a pan of just simmering water. Beat with an electric whisk until hot then remove from the heat and continue to beat continuously for 10 minutes. The mixture will double in volume and should fall in ribbons on the surface when dripped from the whisk.
Melt the butter until it starts to turn golden (called a beurre noisette) then with the whisk running add the melted butter. Sieve in the flour and fold very gently into the mixture with a large metal spoon; be careful not to over-mix as this will prevent the cake rising properly.
Divide the mixture between the tins and bake for 25-30 minutes or until a skewer comes out clean. Cool in the tin for 10 minutes then turn out on a wire rack to cool.
Once ready to serve, beat the cream with the vanilla sugar until light and fluffy. Carefully slice off the top golden crust from each cake. Don’t throw these away - they can be enjoyed as a teatime treat with jam and cream! Cut the sponges in half horizontally to make 4 sponge layers.
To assemble the cake, place the bottom layer on a cake stand or serving plate and spread with a layer of whipped cream. Layer some berries on top, slicing larger ones, then cover with another layer of cream and sit the second sponge on top. Top this sponge with just cream then top with the third sponge. Cover with cream and berries then sit the final sponge on top, cover with cream and berries to decorate. Dust with icing sugar to finish and serve.