1. Grate or roughly chop the chocolate into a bowl. Sit over a saucepan of simmering water until melted.
2. Stir in the rum then remove from the heat. Stir in the egg yolks.
3. Whip the egg whites until a soft peak forms. Gently fold into the chocolate.
4. Whip the cream and fold in. Pour the mousse into individual serving dishes and refrigerate until set.
5. Peel and segment the oranges. Heat a pan, add the sugar and squeeze the remaining juice from the oranges, adding enough extra orange juice to make a syrup. 
6. Simmer the oranges for 5 minutes then let cool. Sprinkle with the lavender before serving with the mousse.

Chef's tip : When melting chocolate, roughly chop or grate it first. Chocolate needs to be melted over a gentle heat, preferably in a bowl over simmering water. If too hot the chocolate will split and burn. A microwave works well as long as you heat gently. Chocolate only needs to be warm to melt.