Frozen berry slice

Preheat the oven to 180°C (160°C, gas mark 4). Line an 18 x 28 x 4cm (7 x 11 x 1½ inch) loose bottomed rectangular cake tin with non-stick baking paper, snipping diagonally into the corners of the paper and pressing into the tin so that the base and sides are lined.

Add all the sponge ingredients to a bowl or food processor and beat together until smooth. Spoon into the paper lined tin and smooth into a thin even layer.  Bake for 15 minutes until golden and the sponge springs back when lightly pressed with a finger. Leave to cool in the tin.

Meanwhile, make the mousse by adding the water to a small heatproof bowl; sprinkle the gelatine over the top so that all the dry powder is absorbed. Leave to stand for 5 minutes then stand the bowl in a saucepan of hot water and simmer gently for about 5-10 minutes until the gelatine is a clear liquid.

Puree the strawberries and raspberries in a food processor or liquidiser then press through a sieve. Lightly whisk the cream in a bowl until it forms soft swirls then whisk in the condensed milk and lemon juice until thick and smooth. Whisk in the fruit puree then gradually fold in the gelatine, pouring it into the bowl in a thin steady stream.

Pour the mousse over the sponge base.  Freeze overnight, loosely wrapping in foil when set.

To make the sauce, puree the berries then press through a sieve, pour into a plastic box and chill in the fridge overnight.

When ready to serve, remove the dessert from the freezer.  Unwrap, remove from the tin and peel away the lining paper.

Cut into 10 bars with a knife dipped in hot water.  Transfer to serving plates.  Arrange extra berries on top, dust with sifted icing sugar and serve with a separate jug of pureed berries.