1. Place mixture bowl and beater attachment in freezer.
2. Using a double boiler heat the chocolate, espresso, rum and Plugrá Butter until melted, the mixture will be smooth and glossy. Remove from heat. Make sure that you stir the mixture during the melting process to ensure hot spots do not occur.
3. Place the cold water in a shallow pan or bowl, immerse the gelatin sheets in to the water. The gelatin will need to sit for approximately 4 minutes or until it becomes soft and pliable. Remove the excess water by wringing the water out of the gelatin sheets by hand. Add the gelatin sheets to the scalded cream and stir until completely dissolved.
4. Place ½ a cup of the heavy cream in a small heavy bottomed saucepan. Heat the cream to the scalding point.
5. Stir the gelatin mixture into the melted chocolate.
6. Remove the bowl and beater attachment from freezer and add the remaining cream. Beat cream until soft peaks form. Fold the cream in stages into the chocolate mixture, making sure not to deflate the cream by over mixing.
7. Whip egg whites in a separate bowl to medium peaks. Carefully fold the whites into cream/chocolate mixture, until just incorporated.
8. Pipe or spoon mousse into desired serving dishes and refrigerate until set, approximately 1 ½ hours.

Chef's Note
To make a firmer mousse that remains soft in texture but will still hold its structure omit the egg whites