Sponge Cake with Chocolate Frosting

For the batter: Separate eggs. Combine yolks with half of sugar, salt and lemon zest and beat until fluffy. Beat egg whites until stiff, then gradually add remaining sugar, beating constantly. Gently fold egg whites into egg yolks. Combine flour with cornstarch and gradually add to egg mixture. Gently fold in butter. Line a loaf pan with parchment paper. Pour batter into loaf pan and bake in preheated oven at 200°C (approximately 400°F)  for about 35 minutes. Cake is ready when a toothpick inserted in middle comes out clean. 

Remove cake from oven and cool briefly. Invert onto plate and carefully remove parchment paper (if necessary, brush with a little water to remove paper easily). Cool on a wire rack. Drizzle liqueur on the cake and allow to soak.

For the chocolate frosting: Combine milk, sugar and chocolate pudding and prepare according to package instructions. Cool. Melt dark chocolate over a hot water bath. Beat butter with powdered sugar until fluffy, then add melted and slightly cooled dark chocolate. Refrigerate about half of frosting. Coat cake with remaining frosting. Fill a piping bag fitted with a plain tip with cold frosting and decorate cake as desired. Refrigerate for at least 2 hours.