Combine ingredients for chocolate whipped cream in bowl and refrigerate for 1 hour to let cocoa dissolve.
Preheat oven to 350F/180C.
Coat a 17 x 12 inch(43 x 30cms) baking pan with butter generously. Line pan with a sheet of butter paper, butter thoroughly and flour the paper.
Melt the chopped chocolate in a pan set over simmering water. Stir smooth and cool.
Separate eggs, placing whites and yolks in separate bowls.
Beat the egg yolks with the ¼ cup sugar for 5 mins till it doubles in volume and forms a ribbon when the beaters are lifted. Scrape bowl often while beating.
Add vanilla extract and beat in.
Add the melted chocolate and beat only to combine & set aside.
With clean beaters, beat the egg whites till foamy. Add the vinegar and beat to soft peaks.
Sprinkle the 2 Tablespoons sugar over and beat till firm peaks form.
Stir a generous spoon of the beaten egg whites into the egg yolks to lighten the yolk mixture. Then fold in the remainder of the egg whites gently & thoroughly.
Spread the batter evenly in the pan.
Bake for 15 – 17 minutes, till the cake has puffed up, lost its shine and springs back when touched.
Remove pan from oven and place on rack to cool. Cover cake lightly with a sheet of butter paper.
Beat the chocolate whipped cream to stiff peaks using clean beaters.
Spread the cooled cake with whipped cream. (Keep 2 tablespoons aside if forming a tree branch as given below)
Gently roll up cake from the short side and place seam side down on serving dish.
If desired, cut a 2 inch thick slice off one end and attach to the side of the cake using the reserved whipped cream, to resemble a tree branch.
Fill a sieve with icing sugar and tap over cake to coat generously and resemble freshly fallen snow.
Keep chilled till served.