Preheat the oven to 220C/425F/Gas 7. Line two 45cm x 30cm/18in x 12in baking trays with greaseproof paper and brush with the melted butter. Line the base and sides of a 25cm/10in springform cake tin with greaseproof paper.
To make the joconde paste, cream the butter and sugar until light and fluffy then gradually add the egg whites, beating continuously. Fold in the sifted flour then add the food colouring. Spoon the mixture into a piping bag fitted with a 5mm/¼in plain nozzle. Pipe the mixture onto the baking trays in a swirl pattern.
To make the joconde sponge, whisk the egg whites until soft peaks form when the whisk is removed. Add the granulated sugar and continue whisking until stiff peaks are formed. Cover with cling film to prevent the meringue collapsing.
Beat the almonds, icing sugar and eggs in a bowl until the mixture is light and fluffy. Mix in the flour and cocoa powder.
Gently fold in the meringue mixture using a large spatula. Mix a cupful of the sponge batter into the clarified butter, then fold this into the remaining batter.
Remove the baking trays with the decorated sheets of greaseproof paper from the freezer. Divide the batter evenly between the two baking trays, spreading it smoothly over the decorations and ensuring it is level.
Bake for 5-7 minutes or until the sponges are lightly browned. Cover each sponge with a sheet of greaseproof paper then upturn the baking trays onto the work surface and peel off the paper to reveal the pattern.
Cut strips of sponge to line the sides of the cake tin, ensuring the pattern is facing outwards, then cut a circle of sponge to line the base. Cut a second circle to make the top of the cake.
For the mousse, pour the orange juice through a fine sieve into a small bowl and sprinkle over the gelatine. Set the mixture aside for three minutes then place the bowl over a small pan of simmering water, taking care the bottom of the bowl does not touch the water, and stir gently until the gelatine has dissolved.
Meanwhile, place the chocolate in a large bowl and melt it in the microwave, stirring every 45 seconds. Mix the orange zest and egg yolks into the melted chocolate, then stir in the gelatine mixture and fold in the whipped cream.
Whisk the egg whites until they form stiff peaks then gently fold into the chocolate mixture. Pour the mousse into the cake tin, place the sponge circle on top and chill in the fridge for at least two hours.
For the orange gelée, mix the arrowroot with a little orange juice and stir into the rest of the juice. Heat in a saucepan until the mixture thickens and clears then remove from the heat and set aside to cool. Carefully remove the cake from the tin and place onto a serving plate. Cut thin strips of peel from the oranges and curl them around a straw. Pour the cooled gelée over the mousse cake, decorate with whipped cream and orange zest curls and dust with cocoa powder.