Step 1Heat the oven to 350 degrees. Generously grease a baking sheet or jellyroll pan. Line it with parchment paper. Generously grease the parchment paper (this will help release cake later).
Step 2Place the eggs still in their shells in a bowl of hot water to warm for 1 to 2 minutes. Do not leave them in any longer, and do not make the water so hot that it cracks the eggs.
Step 3Fill a mixing bowl with very hot water to warm it, then dry it completely. Break the eggs into the warmed bowl and mix them with the whisk on slow speed just to break them up. Increase the speed to high and beat the eggs 10 minutes, dusting in the sugar gradually as eggs are being whipped. After 10 minutes, the batter should be extremely voluminous.
Step 4In a small bowl, sift together the cake meal, potato starch, cocoa, vanilla sugar and salt.
Step 5Gently transfer the batter into a very large mixing bowl. Fold in the dry ingredients in small increments, gently folding after each addition. Drizzle in the melted margarine or oil, taking care not to deflate the mixture too much (some deflation is impossible to avoid).
Step 6Pour the batter into the prepared pan and smooth it on top. Bake until the cake is set in the center or until the cake seems just firm when lightly touched, 35 to 40 minutes. Cool about 10 minutes, then remove the cake from the baking pan (with the parchment paper still attached). Using the parchment paper as an aid, roll up the cake into a jellyroll or log. Wrap the log in a new sheet of parchment paper and wrap the whole thing in a clean kitchen towel. Let the cake rest this way 20 to 30 minutes to allow the cake to set and retain its log shape. (It will be filled and re-rolled later.)
Step 1Cream the margarine with the cocoa and most of the powdered sugar on slow speed. Increase the speed to high, stopping occasionally to scrape down the bowl, and whip to make a fluffy frosting, adding a couple of tablespoons of water (or coffee or cola) as required to achieve proper consistency, 2 to 4 minutes.
Step 1Gently unroll the cake and carefully remove the parchment paper. Spread on the chocolate butter cream filling with a metal spatula. Re-roll the cake into a log and place it seam-side down. Trim about 1/2-inch from each end to make it neat.
Step 2Melt the chocolate chips in the microwave 2 to 3 minutes. Stir the chocolate to make it smooth. Spread it as evenly as possible over the cake using a pastry brush or small metal spatula. Garnish the cake with chocolate shavings and a dusting of powdered sugar or with toasted slivered almonds and powdered sugar.