Heat oven to 180C/160C fan/gas 4. Beat the butter with the sugar, then beat in the flour, baking powder, vanilla and eggs. Pour the batter into a lined 24cm loose-based cake tin and bake for 30 mins or until risen and cooked through.
Remove the cake from the tin and cool on a wire rack, leaving the base paper on the bottom of the cake. When the cake is completely cool, cut it in half horizontally. Clean the tin and line with cling film. Put the top of the cake into the base of the tin.
To make the mousse, put the berries in a pan over a low heat and bring gently to a simmer, you shouldn’t need any extra liquid as they release juice. Sweeten to taste with icing sugar - you want a slightly tart flavour as they will be later mixed with a sweet meringue. Whizz with a stick blender or pop into a blender and puree, then sieve. You should end up with about 250ml puree. Soak the gelatine in water until floppy, stir into the hot berry puree and set aside to cool.
Whisk the egg whites to soft peaks, then whisk in the caster sugar in 4 batches. Continue whisking until you have a stiff, glossy meringue. Fold in the berry puree followed by the cream. Pour onto the sponge base, then carefully lay the other piece of sponge on top, paper-side up. Freeze until 40 mins-1 hr before you need it, then remove from the freezer and peel off the paper. Remove the tin and cling film. When you are ready to serve, dust the top with icing sugar.