Step 1

Separate the base and side of a 21cm (base measurement) springform pan. Lay a sheet of baking paper over the base, overhanging the edge. Clip the sides back on, with the paper hanging out. Trim savoiardi biscuits so that they stand about 1cm above the rim of the pan, reserving the offcuts. Arrange around the edge, cut-side down. Lay a few whole savoiardi biscuits over the base of the pan, as close together as possible, then fill in gaps with the offcuts, trimming to fit (see notes). Don’t worry if there are a few little gaps here and there.

Step 2

For the white chocolate mousse layer, place the white chocolate in a heatproof bowl. Heat 125ml (1/2 cup) of the cream in a small saucepan or in the microwave until it just comes to the boil. Immediately pour over the chocolate and stir with a metal spoon until the chocolate melts and the mixture is smooth. Set aside to cool slightly. Separate 1 egg and stir the yolk into the chocolate mixture.

Step 3

Place 1 1/2 tbs water in a small microwave-safe bowl and sprinkle with 1 tsp gelatine powder. Set aside for 2-3 minutes to soften. Microwave on high for 20 seconds or until warm. Use a fork to whisk until the gelatine dissolves. Set aside to cool slightly. Stir into the chocolate mixture (see notes).

Step 4

Use electric beaters to beat 175ml cream in a bowl until soft peaks form. Use clean, dry electric beaters to beat 1 egg white in a separate bowl until soft peaks form. Use a metal spoon to gently fold the cream and egg white into the chocolate mixture until just combined. Pour into the prepared pan and smooth the surface. Place in the fridge to chill for 2 hours or until just set.

Step 5

For the dark chocolate mousse layer, follow the same method for making the white chocolate mousse layer, using the dark chocolate and the remaining 300ml cream, 1 egg white and 1 tsp gelatine powder. Pour the dark chocolate mousse over the white chocolate mousse layer and smooth the surface. Cover and place in the fridge to chill for 6 hours or overnight or until set.

Step 6

Just before serving, transfer the cake from the pan to a serving platter and slide the paper out from underneath. Use electric beaters to beat the extra 300ml cream in a bowl until firm peaks form and dollop on top of the cake. Use a vegetable peeler to shave the extra dark chocolate and sprinkle over the cream. Cut into wedges to serve.