Melt the chocolate and butter together over a double boiler. Use a large bowl if possible or transfer to one once melted.
Let the mixture cool to room temp then briskly stir the egg yolks into the chocolate mix to avoid lumps. Set aside
Meringue Base
Whip the egg whites till foamy then add the cream of tartar
Keep whipping until soft peaks form, i.e. a beater lifted out of whites will create a peak that folds over the top
Add the sugar in 1 tbsp at a time, mixing for a minute between additions to incorporate.
Keep beating until stiff peaks form, i.e. a beater lifted out of the meringue will result in a stiff peak of meringue
Cream
Beat the cream and sugar together till the cream is fluffy, a beater lifted out of the cream mix should see distinct peaks form
Putting it Together
Fold about a quarter of the meringue into the egg yolk mixture to lighten it. When folding take care to gently swoop under and over to mix and not stir.
Now gradually fold in the remaining meringue in two more batches taking care to be gentle to help maintain texture.
Fold in the cream, refrigerate until ready to serve.
Notes
-Folding egg whites can be a little tricky because you don't want to deflate that beautiful volume. To read a quick how to click here