Line one 10-inch cake pan with parchment paper. Preheat the oven to 340 degrees F (170 degrees C).
Sift the flour and corn starch into a bowl, reserving 1/4 teaspoon of corn starch for later. Mix in the minced chocolate chips and set aside.
Mix the marzipan, vanilla seeds, lemon zest and 1 egg yolk. Add the rest of the egg yolks gradually and whisk until fluffy.
Beat the egg whites, sugar and salt until stiff peaks form. Add in the 1/4 teaspoon of corn starch and beat the mixture briefly. Fold a third of the egg white into the marzipan mixture, then gradually add the rest and mix. Add the flour and chocolate mixture last and combine well.
Pour batter into the prepared cake pan and bake for 40 minutes at 340 degrees F (170 degrees C). Let the sponge cake cool and remove it from the pan.