For the batter: Beat yolks, 1 tablespoon sugar and lemon zest until fluffy.
Beat egg whites with remaining sugar until stiff and fold into egg yolk mixture, then fold in flour.
Spread batter on a greased baking sheet and bake in an oven preheated to 180°C (approximately 350°F) for about 12 minutes. Remove, let cool slightly, release from pan onto a tea towel dusted with sugar. Peel off parchment and cover with a damp tea towel.
Coarsely chop chocolate and melt over a double boiler, then let cool slightly.
For the filling: Beat butter and powdered sugar until foamy, remove 4 tablespoons and set aside. Stir chocolate and cognac into remaining butter mixture.
Spread filling onto cake and roll up with tea towel.
Pour reserved butter mixture into a piping bag with a star tip, then sue to decorate roll as desired.
For the garnish: Top roll with cherries and chocolate flakes. Slice and serve.