To prepare the genoise:
Grease and line a 9-inch cake tin with parchment paper. Preheat the oven to 350 degrees F.
Whisk the eggs and sugar, preferably with an electric mixer, until tripled in volume, and thick and ribbony, like whipped cream. Add the vanilla. Scatter over the flour and salt, a spoonful at a time, and gently fold it in, without over-mixing. Pour the batter into the pan, and bake until a toothpick inserted in the center comes out clean, about 25 minutes. Let cool in the pan for 10 minutes, before removing to a rack, and cool completely.
To prepare the topping:
Melt the jelly with a the water in a saucepan, and strain. Lay the cake on a rack, bottom up, and brush the jelly mixture over the surface. Roll out the marzipan exactly to the size of the cake, and lay it on top. Trim the edges of the cake, wrap, and chill for half an hour so it will cut neatly.
While the cake chills, make the icing: Put the corn syrup with 2 tablespoons warm water in a saucepan, and heat to dissolve. Beat in the sugar, adding about 2 more tablespoons of warm water (or part liqueur, if you like) to make a smooth icing which, when poured, will drape over the cakes like a satin sheet. Tint the icing with a few drops of food coloring, if using.
Remove the cake from the refrigerator and cut into perfect 1-inch squares with a sharp knife. Place them on a wire rack set on a baking sheet. Spoon the icing over each 'cakelet' to coat completely. Let the icing set, and repeat. Decorate the cakes, and store in an air-tight container until serving.
For a more moist cake, brush a couple of tablespoons of liqueur over the cake before the jam.