Melt the chocolate and butter together in the microwave, or over a double boiler with water in the bottom (Microwave: cook on high, 30 seconds at a time, stirring in-between until melted).
Process egg whites with cream of tartar until stiff (aprox. 2 1/2 minutes).
Put 1 cup of sugar and 9 TBLS of water in a small saucepan and bring to a boil, stirring until the sugar is dissolved.
With the processor running, pour the hot sugar syrup into the beaten egg whites through the tube and process for 60 seconds.
Transfer the contents to a bowl.
Put the egg yolks and 3/4 cups of sugar in the processor and process for 30 seconds. Remove the cover and add the melted chocolate and vanilla. Process just until blended.
Gently add the egg whites from the bowl and pulse until they disappear.
Pour the mixture back into the bowl and use a wire whip to finish blending.
You can put the Mousse into cups, or leave it in the bowl. Refrigerate for at least 4 hours. Serve with whipped cream on top.