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 purée, then sieve. You should end
up with about 250ml purée. Soak the
gelatine in water until floppy, stir into the
hot berry purée 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 purée
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.