Prepare the Batter: Beat eggs and sugar until fluffy. Mix in flour, potato starch, milk, lemon zest, and baking powder to form a thin batter. Pour onto a parchment-lined large baking tray.
Bake: Bake at 300°F for a few minutes until the cake is lightly golden. Lay out a kitchen towel and invert the cake onto it. Gently peel off the parchment. Lightly sprinkle the cake with Alchermes.
Make Pastry Cream: Heat milk with lemon zest in a saucepan. In a separate bowl, whisk egg yolks, sugar, cornstarch, and vanilla. Gradually add some hot milk to the egg mixture, then return all to the saucepan. Cook over low heat, stirring, until thickened. Remove from heat and cover with plastic wrap directly on the surface. Cool.
Assemble: Spread the pastry cream evenly over the cake. Carefully roll the cake using the towel, forming a log about 4 inches in diameter.
Finish and Serve: Place the roll on a serving tray and dust with powdered sugar. For variations, fill with whipped cream and melted chocolate, whipped cream and fresh strawberries, or chocolate cream.