Place eggwhites in a bowl. Using an electric mixer, beat until soft peaks form. Using a metal spoon, stir one-third of eggwhites into chocolate mixture. Gently fold remaining eggwhite through chocolate mixture.
Pour mixture into prepared pan. Bake for 1 hour or until a skewer inserted into the centre has moist crumbs clinging. Stand in pan for 10 minutes. Turn out onto a wire rack to cool. Slice in half length wise.
To make the white chocolate mousse, start by melting your chocolate on a double boiler. Transfer to a bowl and set aside to cool a bit. In the meantime, whip up 1/2 cup of heavy cream until you get whipped cream. Take a spatula and slowly fold in the melted white chocolate, making sure not to deflate the whipped cream. Once the chocolate is mixed in, put the mousse in the fridge for 30 minutes.