PASTRY: Combine flour, icing sugar and 1 melted square chocolate.
Cut in 1/2 cup softened butter (with pastry blender or in food processor) until mixture resembles coarse crumbs and form into ball with your hands. Press into bottom and sides of flan ring pan with removable bottom that has been greased and chill for 1 hour.
Bake at 425 for 10 minutes and then cool.
MOUSSE FILLING: Bring 1/2 cup whipping cream to a boil and add 5 squares of chocolate and 2 tablespoons butter; stir until melted.
Blend in 2 eggs yolks well and remove from heat.
Beat egg whites until frothy, gradually add 1/4 cup sugar, beating until stiff peaks form.
Gently fold into chocolate mixture; pour into pastry crust, chill 10 minutes.
GARNISH: Shortly before serving, sprinkle 3 squares (coarsely grated - use a vegetable peeler to make curls) over mousse.
Lay 3 strips of wax paper across flan.
Sift icing sugar over flan; remove wax paper.
NOTE: Don't make or serve this on a hot day; it will melt.