In the top of a simmering double boiler place the white chocolate and melt it. Keep it warm. In the top of another simmering double boiler place the white chocolate and melt it. Keep it warm. Add the melted chocolate to the egg yolk and stir it in. In another small saucepan place the gelatin and the water. Heat them slightly and mix them together so that the gelatin is totally dissolved. Let the mixture cool to room temperature. Add it to the egg-yolk-chocolate mixture and stir it in well. Carefully fold the whipped cream into the mixture. Place the mousse in the refrigerator for 1 hour, or until it is firm.