FOR THE POPPY SEED SHEET CAKE: Grease 11 x 17 inch baking sheet (28 x 43 cm). Line bottom with parchment (NOT WAXED) paper, extending paper 2 inches over long sides.
Heat milk in small saucepan on medium for about 4 minutes until very hot and bubbles start to appear around edge. Stir in lemon juice, poppy seeds and lemon zest. Let stand for about 15 minutes until cooled to room temperature.
Beat 1 cup butter in large bowl until creamy and light coloured. Add both sugars (granulated and Icing). Beat well. Beat in egg yolks and vanilla.
Stir flour, baking soda and salt in medium bowl. Add to the butter mixture in 3 parts, alternately with poppy seed mixture in 2 parts, beating well after each addition.
Beat egg whites in small bowl with clean beaters until stiff (not dry) peaks form. Do not overbeat. Gently fold 1/2 of egg white into poppy seed batter. Fold in remaining egg white until no white streaks remain. Spread evenly in prepared baking sheet.
Bake in centre of 350°F oven for about 20 minutes or until wooden pick inserted in centre comes out clean. Let stand in baking sheet on wire rack until cooled completely. Run knife along side to loosen. Invert cake onto counter. Carefully peel off and discard parchment paper. Trim off crusty edges all around. Cut cake crosswise into 3 rectangles, about 4 x 10 1/2 inches each (10 x 26 cm)).
LEMON BUTTERCREAM ICING: Combine granulated sugar and flour in medium saucepan. Whisk in lemon juice and egg yolks. Heat and whisk on medium for about 3 minutes until mixture is hot and sugar is dissolved.
Add first amount of butter (6 tbsp) 1 piece at a time, stirring until melted. Heat and stir until just boiling and thickened. Whisk in lemon zest and whipping cream. Turn into medium bowl. Cover with plastic wrap directly on surface to prevent skin from forming, Chill until cold.
Beat second amount of butter (2 2/3 cups) in large bowl on high for a bout 5 minutes until light coloured and increased in volume.
Gradually add 2 1/4 cups icing sugar (confectioner’s) 1/4 cup at a time, beating well after each addition. Add lemon mixture, 1/4 cup at a time, beating until creamy and light coloured. Makes about 5 cups icing.
Place 1 cake piece on long 12 inch serving plate or cake board. Spread 3/4 cup icing on top. Repeat with second and third cake pieces and same amount of icing each time. Place remaining cake piece on top. Ice top and sides with very thin layer of icing. Spoon remaining icing into large piping bag fitted with small closed star tip. Pipe small stars on top and sides of cake. Chill for several hours until firm for easier slicing. Cuts into 24 slices.