1 For the base: heat the oven to 180°C (160° fan) 350°F gas 4. Grease a 23cm|9" springform cake tin and line the base with non-stick baking paper.
2 Stir the butter and chocolate in a pan over a low heat until melted and smooth. Cool to room temperature.
3 Whisk together the eggs and sugar in a mixing bowl until thick, then whisk in the vanilla.
4 Stir in the chocolate mixture. Sift in the flour and stir in gently.
5 Put into the tin and bake for 15-20 minutes until a skewer inserted into the centre comes out with dry crumbs attached. Cool completely in the tin on a wire rack.
6 For the mousse: stir the white chocolate and 175ml|3/4 cup cream in a pan over a low heat just until the chocolate is melted. Cool completely.
7 Whisk the remaining cream until peaks form.
8 Whisk the egg whites until stiff but not dry.
9 Gently fold the egg whites, then the whipped cream into the white chocolate mixture.
10 Spoon the mousse on top of the base in the tin and smooth the top. Cover and chill overnight.
11 Remove from the tin. Decorate with orange slices and orange zest, as in the photo.