To make the sponge:
Preheat oven to 350 degrees F. Prepare two baking sheets by greasing them and lining with parchment.
Sift the matzo cake meal, potato starch and salt together, set aside. In a mixer whip the eggs and sugar, for about 8 minutes.
The eggs should be very light in color and a thick foam. At the very end, add the vanilla and zest.
Sift the dry ingredients over the egg foam.
Very gently, but thoroughly, fold the dry ingredients into the egg foam.
Pour the batter into one of the prepared pans and spread evenly with a spatula.
Bake the cake for about 18 minutes or until golden and dry when poked with a cake tester. Repeat with the second cake layer.
To make the buttercream:
In a small pot, fitted with a candy thermometer, heat 2 cups sugar, water and cream of tartar to 242 degrees F. This will take several minutes, so get your eggs started.
Whisk together the eggs, yolks and 1/3 cup sugar. Beat the eggs on medium high speed until light in color and thick.
Once the sugar has reached 242 degrees F, add it very carefully and slowly to the eggs, while the beater is going on slow speed. Be sure to pour the syrup along the edge of the bowl, not directly onto the beater or it will splatter. Continue beating the egg mixture on medium-high speed until the eggs have cooled, about 8 minutes.
Once the eggs are cooled, add the butter 2 tablespoons at a time. The mixture will go through a stage when it looks soupy and curdled, but keep adding the butter and it will come together. After all the butter is added, mix in the vanilla.
The buttercream will be smooth and glossy.
Divide 1/3 of the buttercream into a separate bowl and add the poppy seed paste.
To the remaining buttercream, add the melted chocolate.
If you are not going to use the buttercream right away, cover it and leave it at room temperature for up to 24 hours. When you are ready to use it, give it a good stirring or put it back in the mixer and beat with the paddle attachment until smooth.
To make the soaking syrup:
In a small saucepan heat the sugar, water and zest over medium heat until the sugar has fully dissolved.
Cut each sheet of cake into 4 equal pieces (they will be about 4 inches wide and 12 inches long).
Lay the first layer of cake on the serving platter and brush with the soaking syrup. You want to dab the syrup on, but don't overly saturate the cake or it will be soggy.
Spread a thin layer of the poppy seed buttercream over the soaked layer.
Repeat with 5 more layers and then finish with the 7th layer of cake. This will leave you with one extra. Since my cake is not traditional anyway, you should feel free to use up that last layer. If you want to keep it to seven layers, then I suggest you use that lonely extra layer as a snack.
If your layers need squaring up, just trim them with a sharp serrated bread knife.
Stir the chocolate buttercream to make sure it is smooth and cover the entire cake in a nice even layer.
It is okay if it isn't perfectly smooth, because we're going to add some rough stripes to the outside.
Take a small blob of buttercream on the end of your spatula,
and starting at the end of the cake, on the bottom, spread that blob in a stripe along the cake. Repeat over and over again, but alternate which side you start on. Repeat this on the top as well.
You will end up with rough stripes all over the cake, which gives the cake some texture.
Using a sharp chef's knife, scrape the smooth side of a bar of chocolate with the blade, so that the chocolate curls up. If the chocolate is very cold and brittle, try sitting it in the sun for a few minutes and the curls will come more easily. Cover the top with the chocolate shavings.